The literal meaning of Mahajanapadas is great kingdoms. They flourished in the north/north western parts of India before the rise of Buddhism. Aryans have migrated into India long time back and there were regular friction between them and the non aryan tribes concerning, cattle, fodder, land etc. These tribes of Aryans were called as Janas by many Vedic texts. Later on there was a merger of the Vedic Janas into Janapadas. Different regions of the Indian subcontinent were previously divided into Janapadas, this was a clear demarcation by boundaries. Many Janapadas by 600 BCE further developed into bigger political bodies. These kingdoms came to be known as Mahajanapadas in the Buddhist traditions.
Sixteen great kingdoms as they are referrd to by buddhist and other texts. The sixteen mahajanapadas include Kasi, Kosala, anga, Magadha, Vajji, Malla, Chedi, Vatsa, Kuru, Panchala,Machcha, Surasena, Assaka, Avanti, Gandhara and Kamboja.
Out of the above 16 states Kuru, Panchal, Shursen, Vats, Kaushal, Malla, Kashi and Chedi were present in Uttar Pradesh and are still in the state. More known among them were Kaushal, Kashi and Vats beides these certain republican states were also within the boundary of Uttar Pradesh.
Kasi:
The name Kasi is the tribe who settled in the region around Varanasi where itself the capital was located. There is a belief that Varanasi got its name from the rivers that surround the city, namely Varuna and Asi. Kasi occupied a predominant position among the sixteen Mahajanapadas, before the rise of Buddha. We come to know a lot about Kasi from the Jatakas which were a voluminous body of myths and folklore revolving about prvious births of the buddha. This supremacy called for a long drawn conflict for mastery between other cities, like Kosala, Anga and Magadha with Kasi. Kasi was no doubt influencial that is the reason why we get a mention of Kasi in the Vedic texts. Matsya Purana and Alberuni are the texts where we read Kasi as Kausika and Kaushika, others read it as Kasi.
Kosala:
Among the sixteen Mahajanapadas, Kosala is one, which comprised of Shravasti, Kushavati, Saket and Ayodhya. Kosala constituted of the territories of modern Oudh or Awadh which is located in Uttar pradesh. The state capital of Kosala was Ayodhya which was under the command of Prosenjit the Kosala King, a contemporary of Gautama Buddha. The southern side it was bordered by the Ganges, the east had river Gandhak encircling it. Magadha was a neighbouring state to Kosala, and there were conflicts between them. Ajatshatru who was the king of Magadha and Prasenjit were in continuous struggle for power which finally came to an end with the alignment of the confederation of Lichchavis with Magadha. After Prasenjit, Vidudabha rose into power and Kosala ultimately amalgamated into Magadha.
Anga:
India's earliest empire was evolving around the Gangetic plains, which included the Mahajanapadas. Anga was one of these evolving states, which is one of the sixteen Mahajanapadas that prospered during that period. Malini, Champapuri, champa Malini, Kala Malini etc were the different names by which this sate was called. The Angas were first referred to in the Atharva Veda as the detested people. Atharva Veda considers Anga an unholy place and some even condemned it as a place where wives and children were sold. Mahabharata, testifies the people of Anga to be of noble birth or 'Sujati' proclaiming the sanctity of the place Champa as a pilgrimage. During the reign of Bimbisara, this Mahajanapada was usurped and taken over by Magadha. Champa was also a major seat for the spread of Jainism and Buddhism.
Magadha:
Magadha emerged as a powerful kingdom in the reign of Bimbisara and his son Ajatshatru. The earliest ruling dynasty according to Mahabharata and Puranas seems to be founded by king Brihadratha. The Vedas have a mention of the Magadhas as semi 'brahmanised' and this was a reason for the not so good impression of the people. Kikata was a non Aryan country according to Yasaka and the king Pramaganda is said to be the ruler of Kikata. Kikata on the other hand was considered a synonym for Magadha in later Vedic literature.
The city was known by many other names like Magadhapura, Brihadrathapura, Vasumati, Kushagrapura and Bimbisarapuri. Buddhism and Jainism were in vogue in the religious scenario during that time, and Magadha became a dynamic center of Jainism along with the first Budhist Council being held in Rajagriha in the Vaibhara Hills.
Vajji or Vriji:
Sixteen Mahajanapadas of ancient Inida includes Vajji as one of them. The Vajji was a confederation a many clans of which the Licchhavis, the Vedehans, Jnatrikas and the Vajjis were the most important. It was actually known as the Vajji Sangha or the union of Vajji, which comprised of many janapadas, gramas (villages), gosthas (groups). The eminent people were chosen from each khandas (districts) to represent on their behalf in Vajji gana parishad (people's council of Vajji). The chairman of the council was called Ganapramukh (head of the democracy), but often he was addressed as the king.The other executives were Mahabaladhrikrit (equivalent to the minister of internal security), binishchayamatya (chief justice), dandadhikrit (other justices) etc. Vajji had its capital at Vaishali.
Malla:
Malla was an ancient dynasty in India and is one of the sixteen mahajanapadas. Epics like Mahabharata mentions that the Mallas were considered along with the tribes of the Angas, Vangas and Kalingas. Buddhist and Jain works have the mention of the Mallas who existed in a republic that consisted of nine teritories. In a more original context it is evident that they actually had a monarchical form of government in the beginning but later they transformed into the republic form{Samgha). The Mallas were very warlike and brave people and have been mentioned and referred as Vrtaya Kshatriyas by Manusmriti, as Vasishthas in the Mahapparnibbana Suttanta. Mallas have also suffered domination by the Magadha empire after Buddha's death.
Chedi or Cheti:
The Chedis were group of ancient people of India living on the south of the river Yamuna. They are mentioned in the Rigveda, and city called Suktimati is mentioned as the capital of Chedi. Chedi kingdom was one of the sixteen Mahajanapadas, and was ruled by Sisupala, an ally of Jarasandha of Magadha and Duryodhana of Kuru. Prominent Chedis during Kurukshetra War included Damaghosha, Shishupala, Dhrishtaketu, Suketu, Sarabha, Bhima's wife and so on. Chedi was the place that was chosen for spending the 13th year of exile by the Pandavas.
Vamsa:
The Vamsa or the Vatsa was the kingdom that followed the monarchical form of government. This kingdom is one of the sixteen Mahajanapadas, and the capital of this was located at Kausambi. One very important aspect of this city was that it formed the hub of all economic activitioes and had a prosperous trade and business relations. 6th century Bc has the account of Udyana to be the ruler of the, kingdom at the time of Buddha. About Udayana it is said that earlier there were resentments on his side regarding Budhism as he was very warlike and aggressive but in the later years became more tolerant and finnaly a folower of Buddha. So much he was affected by his teachings that he made buddhism his state religion.
Kuru:
The kuru janapada is one of the sixteen mahajanapadas. Regarding the origin of the Kurus it has been said that they belong to the Puru-Bharata family. Kurus were the specific origin of people living in the Kurukshetra and according to the Buddhist text Sumangavilasini, the kurus came from the Uttarakuru. Testified by the Vayu Purana, the founder of Kurukshetra or kuru janapada was Kuru who was the son of Samvarsana of the Puru lineage. During sixth/fifth century BCE, the Kurus are believed to have shifted to republic form of government.
Panchala:
Panchala was divided into Uttara-Panchala and Dakshina-Panchala. Counted among the sixteen Mahajanapadas, the northen Panchala had Chhatravati as its capital and the south had its capital at Kampilya. In Panchala is situated the renouned city of Kanyakubja. Like many other kingdoms it was seen that the Panchals tooo had shifted to a republican form of government in sixth and fifth century BCE from being a monarchy.
Machcha or Matsya:
The Kingdom of Matsya was again an important part of the sixten mahajanapadas. This lay south of the Kurus and west of the Yamuna which separated them from the Panchalas. The Machcha tribe inhabited this region which had its capital at Viratanagara. The Matsyas are generally linked up with the Surasenas in Pali literature. The Matsya tribe in comparison to the other janapadas were of not much poolitical emminence during the age of Buddha. Matsyas and the chhedis have a connection here when we see that they were once ruled by the same king Sujata, and Matsya was a part of the kingdom ofn Chedi.
Surasena:
The kingdom of Surasena, underwent a lot metamorphosis in terms of religion. The capital which was Mathura, was the centre of Krishna worship at the time of Megasthenes. Whereas Avantipura who was the king of Surasena was one of the first desciples of Buddha, and it gained prominence evr since then in Mathura. The geographical locattion of this kingdom among the sixteen mahajanapadas was south west of Matsya and west of the river Yamuna. There were various tribe that in habited the region and they were headed by a chief.
Assaka or Ashmaka:
Kingdom of Assaka or Ashmaka was situated in the southern part of India and one of the sixteen mahajanapadas. The Ashmaka had its capital located at Potana or Potali which have resemblences of Paudanya of Mahabharatha. The Assakas are placed in the north-west in the Markendeya Purana and the Brhat Samhita. There are numerous associations regarding the identification of assakas. That is why we have different views on this. Like the commentator of Akutilya's Arthashahstra identifies it with Maharashtra.
Avanti:
Avanti was an important kingdom of the sixteen mahajanapadas, and it lay in the western part of India. Buddhism rose to its prominence in this kingdom and and this was one of the other kingdoms which initiated Buddhism in a larger manner. The kingdom was divided as north and south Avanti and the north had its capital at Ujjaini. Mahissati was the capital of Avanti in the beginning which was integrated into Ujjaini during the period of Mahavira and Buddha. Avanti in the later stages of historu was amalgamated into the Magadha empire under the reign of Shishunaga.
Gandhara:
The Gandhara kingdom comprised of the Gandharas who were highly trained in the art of war and they have a mention in the Atharva Veda as well . though in the Vedas they are mentioned as the despised people along with some others due to their allegiance to non Aryan group. Puranic and Buuddhistic tradition included Gandharas in Uttarapatha. The Gandhara kingdom of the sixteen mahajanapadas was founded by Gandhara, son of Aruddha who was the son of Yayati. It was alos believed once according to Gandhara Jatakas that they they were a part of Kashmir. Gandhara was an important seat of international commercial activities, and provided communication with other countries like Iran and Central Asia.
Kamboja:
Kamboja was believed to have composed of parts that were o the either side of the Hindukush. Whereas originally they were located somewhre else. The Kamboja Mahajanapada of the Buddhist traditions refers to the 'cis-Hindukush branch' of ancient Kambojas. The kamboja being one of the sixteen mahajanapadas were a republic since ages. There are many evidence from the Mahabharata, Kautiliya's Arthashastra and Ashoka's Edict No. XIII which affirms that the Kambojas were a republic people.
Magadha emerged as a very powerful mahajanapada with time and this marked the annexation of sevaral janapadas of the 'Majjhimadesa'. The Kasis, Kurus, Panchalas, Vatsyas etc were certainly among the exterminated clans which had no trace in the folklore, poetry and so on. The sixteen Mahajanapadas were infact distinguished as the ones belonging to the Majjhimadesa or mid India, or Uttarpatha or the north-west region.