What are groups in SFDC ?

Hii Ayush,
A group consists of a set of users. A group can contain individual users, other groups, or the users in a particular role or territory. It can also contain the users in a particular role or territory plus all the users below that role or territory in the hierarchy.

Groups are sets of users. It can contain individual users, other groups, the users in a particular role or territory, and/or the users in a particular role or territory plus all of the users below that role or territory in the hierarchy.
There are two types of groups:
Public Groups: Administrators and delegated administrators can create public groups. Everyone in the organization can use public groups.
Personal Group: Each user can create groups for their personal use.