Permission levels are collections of permissions that allow users to perform a set of related tasks (that is, the read permission level includes the ability to view items, open items, and download items, but not upload them back, view pages, and view versions). Each of the permissions can be included in more than one permission level.
It is best practice to assign individual users to groups, then assign appropriate permissions to groups depending on where the group is being used and what level of access they should have there. A group is a set of users that are defined at the site collection level.
You must consider a user's group membership carefully because a user's permissions determines what that user will ...
