The SharePoint Group report lists all of the groups that are referenced within your SharePoint Site Collections. You can list your groups by Site Collection by selecting the site and clicking Apply on the top left.
The default view for this report shows the name of the Site Collection, the group name, the group description, the group owner and the number of members within each group.
Clicking the Site Collection name will give you a complete breakdown of this Site Collection.
If you select the group you will get a full list of all of the members and the member type.