The Jats/Jatts (Hindi: जाट,Punjabi: ਜੱਟ, Urdu: جاٹ) of Northern India and Pakistan, are descendants of Indo-Aryan tribes.
In India, they inhabit the states of Punjab, Haryana, Rajasthan, Delhi, Uttar Pradesh, Madhya Pradesh, and Gujarat. In Pakistan, they are found in the provinces of Punjab and Sindh. In large parts of these areas the Jats are the dominant land owner caste, particularly in Punjab, Haryana, Western Uttar Pradesh & Rajasthan.
The Jats, like most South Asians, are mostly farmers ; however they are also found in many other professions. A large number of Jats serve in the Indian Army, including the Jat Regiment, Rajputana Rifles, Sikh Regiment and the Grenadiers, among others. Jats also serve in the Pakistan Army particularly in the Punjab Regiment.
The Jat regions in India are among the most prosperous on a per-capita basis (Haryana, Punjab, and Gujarat are among the wealthiest of Indian states).
There are theories about the origin of the Jats. Jats are commonly considered to be of Scythian (Saka), Indo-European, Indo-Iranian or Indo-Aryan stock in view of the similar physical features and common practices.
Jats origin is most likely from Jutes or the Goths. Jats have many surnames common to German people even today.
Both, Sir Alexander Cunnigham and Col James Tod agreed in considering the Jats to be of Indo-Scythian stock. The former identified them with the "Zanthi" of Strabo and the "Jatti" of Pliny and Ptolemy; and held that they probably entered the Punjab from their home on the Oxus very shortly after the Meds or Mands , who also were Indo-Scythians, and who moved into the Punjab about a century before Christ. The Jats seem to have first occupied the Indus valley as far down as Sindh, whither the Meds followed them about the beginning of the present era.
But before the earliest Muslim conquests the Jats had spread into Punjab proper, where there were firmly established in the beginning of the eleventh century. By the time of Babar, the Jats of the salt range had been in constant conflict with the Gakkhars, Awans and Janjuas. Tod classed the Jats as one of the great Rajput tribes; but here Cunningham differed from him holding the Rajputs to belong to the original Aryan stock, and the Jats to a late wave of immigrants from the north west, probably of Scythian race.
In 'Punjab Castes', Sir Denzil Ibbetson wrote:
" .... the original Rajput and the original Jat entered India at different in its history. But if they do originally represent to separate waves of immigration, it is at least exceedingly probable, both from there almost identical physique and facial character and from the close communion which has always existed between them, that they belong to one and the same ethnic stock; and it is almost certain that the joint Jat Rajput stock contains not a few tribes of aboriginal descent, though it is probably in the main Aryo-Scythian, if Scythian be not Aryan."
The Arabian traveller Al-Biruni has mentioned that Lord Krishna was a Jat
The most acceptable theory about the origin of the word, 'Jat' is that it has originated from the Sanskrit language word “Gyat” . The Mahabharata mentions in chapter 25, shloka 26 that Lord Krishna founded a federation ‘Gana-sangha’ of the Andhak and Vrishni clans. This federation was known as ‘Gyati-sangh’. Over a period of time ‘Gyati’ became ‘Gyat’ and it changed to Jat.
The other prominent theory of the word's origins is that Jat came from the word Gaut tribal name of some Indo-Aryan tribes of Central Asia (such as those which later became Gauts/Goths and settled in Europe), which was written in 'Jattan Da Ithihas'. It has also been mentioned by Bhim Singh Dahiya. 
According to the historian 'Ram Lal Hala' the word Jat is drived from word 'Yat'. There was a king named 'Yat' in Chandra Vanshi clan who was ancestor of Lord Krishna. The Jats are descendants of King Yat. 'Yat' later changed to 'Jat'.
There are many variations of the term Jat. In the Punjab, the phonetic sound is 'Jutt'or 'Jatt