Site groups are very important topics SharePoint, because it is very important for site administrators to control who has access to their site collections.A Site administrator can specify who can perform which actions in the the site by adding users to SharePoint site groups.
Larger organization may already have active directory groups created that can be added to site groups which can be a great time saver
Here is list of the standard site groups:
- Reader- has red-only access to the site.
- Contributor-Can add content to existing document libraries and lists.
- Web Designer- can create lists and document libraries and customize pages in the website
- Administrator-Has full control of the site.
- Custom Site Group-A site Administrator can actually create a site group and call it whatever he or she wants(Super Reader, Limited Contributer,Power User, and so on).
a Site Administrator can grant special access to web parts within the site (such as a document library), so if really need to be able to add documents to a document library, but are only a member of the reader site group, the Site Administrator can give you enhanced rights to that library.