A group is a collection of users that can share the same permissions on a specific site or content. When you create a group, you always bundle a specific permission level to it. Later, when you want to assign someone that specific permission level, simply add the user to the group.
A SharePoint group is a collection of people—SharePoint users—who have the same permission level. That is, everyone in the group has the same access on your site. Groups let you manage access to your site for many people at the same time. Some advantages of assigning permissions to groups are:
Groups can help simplify permissions management. Instead of managing permissions for hundreds of individual people, you manage permissions for a few groups. This is especially helpful if you have to change site collection administrators for the site. And groups can contain individual users.