As Hindus we know and accept that all human beings are born with different capabilities (an easily verifiable fact) which later in life lead them to pick up different skill sets. It’s these skill sets and their capabilities that give individuals a greater ease at performing certain functions better than others. What the dharmashatras do is classify the four categories of human beings based on their skill set and their abilities to perform certain functions.
While all varnas are to be respected some varnas have traditionally been more respected than others. This is merely due to the nature of their functions in the material world. It’s the same as a cryogenic scientist being more respected than a cryogenic engineer. While both the scientist and engineer are important, it’s always the scientist who gives the basic concept without which the engineer can’t build a cryogenic engine. Taking the same analogy further the engineer would again be more respected than a simple technician, although the job of the technician is not less important in terms of need for production of the engine as that of the engineer. Human society tends to act as a pyramid, the scarcer your skill set is the greater the demand.
Thus Varna is based on the innate quality and nature of the individual, rather than birth.
While Varna relates to abilities and capabilities; jati relates to the tribal origin. Gujjar, Jats, Thevars, Nadars, Nairs, Menon, Bunts are all Jati and not Varna. Traditionally, large groups of tribals associated themselves with a specific profession so the profession became the identity of the tribal groups.
Gotra relates to Ancestry. Gotra is essential during performance of vedic rituals. Gotra is also a genetic marker in most cases, as marriages within the same Gotra is not usually performed.
Kula means family or family background. Kula determines the social standing and social influence of a family in the community.