Yadav is a category consisting of several allied castes which together constitute about 20% of the total population of India, 20% population of Nepal and about 3% population of the planet earth. Yādav is an caste found in India, Nepal, Sri Lanka, Pakistan, Bangladesh, Russia, Middle East and claims descent from ancient king Yadu, the name of one of the five Aryan clans mentioned in the Rig Veda as Panchjanya, meaning "five people", is the common name given to five most ancient Vedic Kshatriya clans. The Yadav caste generally follows Vaishnav traditions, and share Vaishnav Dharmic religious beliefs. They are the worshippers of Lord Krishna or Lord Vishnu. Yadav’s are classified under the Kshatriya varna in Hinduism and remained in power in India and Nepal until 1200-1300AD, before the arrival of Muslim invaders. Two things are common to these cognate castes. Firstly, they claim to be the descendants of the Yadu Dynasty (Yadav) to which Lord Krishna belonged. Secondly, many castes in this category have a set of occupations centering round cattle. Krishna pastimes lends a kind of legitimacy to the pastoral occupations relating to cattle, and as the castes following these occupations are to be found in almost all parts of India, the Yadav category encompasses a whole range of related castes.
According to Vedic literatures, the Yaduvanshis or Yadavs are the descendants of Yadu, the eldest son of King Yayati. From his line was born Madhu, who ruled from Madhuvana, situated on the banks of river Yamuna, which extended up to Saurastra and Anarta (Gujrat). His daughter Madhumati married Harinasva of Ikshvaku race, from whom Yadu was born again, this time being ancestor of Yadavas. Nanda, the foster father of Krishna, was born in the line of succession of Madhu and ruled from the same side of Yamuna. Jarasandh, Kansa's father-in-law, and king of Magadha attacked Yadavas to avenge Kansa's death. Yadavas had to shift their capital from Mathura (central Aryavart) to Dwaraka (on the western coast of Aryavart) on the Sindhu. Yadu was a legendary Hindu king, believed to be an ancestor of the Lord Shri Krishna, who for this reason is also referred to as Yadava. Genetically, they are in Indo-Caucasoid family. A study in East of India shows their gene structure is similar to Brahmin, Kayastha & Rajput living in the same area.
Some historians also seek a connection between Yadavas and Jews. According to their theory, the Greeks were referred to the Jews as Judeos, or Jah deos or Yadavas, meaning people of Ya.
In Russia, many Russians have surname “Yadav”.