How did jatis get formed?

In India, the term "jati" refers to a caste or a sub-group. The Word is derived from the Sanskrit Word for "born into." Members are thus born into a jati, and they stay within that group, marry within it, and sometimes will not even mingle with members of other groups. Although Mahatma Gandhi tried to outlaw the Caste System, it still persists today, although there is certainly more social mobility now than there was when he lived.

If you have studied sociology, you will find that there are many societies with castes; India is one good example. Sometimes, a caste system is based on ties of clan or kinship (one's family history). Other systems emerge from religious teachings, even if the teachings are somewhat ambiguous. For example, in Hinduism, earlier scriptures said little about caste, but by the time of the writing of the Bhagavad Gita, there seem to have been four established groups (Brahmins at the top, then K'shatriyas, Vaisyas, and Sudras) and then a fifth that was considered the lowest of all-- the Untouchables (today called Dalits). Thus, Jatis are groups that flourish even within those castes, often started by a clan group hundreds of years ago, and perpetuated by tradition and custom.