Clan. In the beginning of the creation, when families started, individual sons were treated as of individual families and Gotra. Gotra is mainly used in marriages. Children of the same Gotra do not usually marry. They are somewhat considered like brothers and sisters and brothers and sisters do not marry each other.

The original classification of Braahman was based on 7 ancient priest families. Gotra were the family names derived from the original Rishi, such as Vashishth, Kashyap etc. As the population increased these 7 Gotra began to be considered as Mool Gotra (original Gotra). Sub-divisions of these Gotra were created in hundreds all traceable to original Gotra. The Gotra of Kshatriya and Vaishya were based on the Gotra of their family-priest from ancient times.
[Aangiras, p 127]

Gotra means lineage, descent, family. The Gotra of each family is derived from its founder (Aadi Purush). Manu speaks of eight Gotra, and the Dharm Pradeep speaks of 42 Gotra. Castes, other than Braahman, derive their Gotra and Pravar from their Guru. Pravar mean well-known excellent illustrious. Thus in Jamadagni Gotra, the Pravar are Jamadagni, Aurv and Vashishth. In Bharadwaaj Gotra they are Bharadwaaj, Angiras and Baarhaspatya.

In each Gotra there are usually three or five Pravar Purush (illustrious men). Pravar is, therefore, a  branch of a Gotra named after a distinguished member of it. A person, therefore, has both a Gotra and a Pravar from which he is descended.

If one does not know or forgets ones Gotra, Dharm Shaastra recommend the adoption of the Gotra of the great Rishis such as Jamadagni or Kashyap.

This is because the major Gotra are divided into minor subdivisions known as Gan and Gan themselves are further subdivided into lineages. The term Gotra, however, is often used to refer to the major Gotra, as also to Gan and even these lineages.

Is Shreevastav a Gotra?
No. As the word "Gotra" means "lineage" in the Sanskrit language. Among the Braahman caste, Gotra are reckoned patrilineally. Each Gotra takes the name of a famous Rishi or sage who was the patrilineal forbearer of that clan. And each Gotra is addressed by the suffix 'sa' or 'asa' as relevant. The concept of Gotra was the first attempt among Braahman to classify themselves among different groups. At the beginning, these gents identified themselves by the names of various Rishi, such as Aangiras, Atri, Gautam, Kashyap, Bhrigu, Vashishth, Kutsa, and Bharadwaaj; the first seven of these are often enumerated as Sapt Rishis. There is also variation about Sapt Rishi.

It is to be noted that Vishwaamitra was initially a Kshatriya king, who later chose and rose to become an ascetic Rishi. Hence the Gotra was applied to the grouping stemming from one of these Rishi as his descendants. Hence it means there could only be 8 Gotra. Today we have more than 40 Gotra. How is it? Over the period, the major Gotra were divided into Gan (subdivisions) and each Gan was further divided into groups of families. The term Gotra was then frequently started being applied to the Gan and to the sub-Gan. So it is the Gan, or the Sub-Gan that is  no commonly referred to Gotra.

Over the years, the number of Gotra increased due to: Descendants of original Rishi also started new family lineage or new Gotra. By inter-marriage with other sub-groups of the same caste, and inspired by another Rishi whose name they bear as their own Gotra.

Pravar - can be the same as Gotra mostly. It can be the lineage saints also. Pravar is the number of the most excellent Rishi who belonged to that particular Gotra to which a person belongs. In Vaidik ritual, the importance of the Pravar appears to be in its use by the ritualist for extolling his ancestry and proclaiming, "as a descendant of worthy ancestors, I am a fit and proper person to do the act I am performing."