Power Hour: SharePoint Permissions
Learn how to set permissions on a site, and on individual lists and libraries in the site. We will also cover how to check someone's permissions to specific content on a site.
Default permission levels are predefined sets of permissions that you can assign to individual users, groups of users, or security groups, based on the functional requirements of the users and on security considerations.
This article contains advanced scenarios for customizing site permissions. Most organizations won't need these options. If you just want to share files or folders, see Share SharePoint files or folders. If you want to share a site, see Share a site.