Permission Levels and Permissions

By default sharepoint creates 5 permission levels, each has its own permissions associated.In case, to customize the permissions associated with a specifc level all 3 levels(except limited access and Full Control) can be modified.

Default permission levels in Windows SharePoint Services 3.0

Permission Level	Description
Full Control	This permission level contains all permissions. Assigned to the Site name Owners SharePoint group, by default. This permission level cannot be customized or deleted.
Design	Can create lists and document libraries, edit pages and apply themes, borders, and style sheets in the Web site. Not assigned to any SharePoint group, by default.
Contribute	Can add, edit, and delete items in existing lists and document libraries. Assigned to the Site name Members SharePoint group, by default.
Read	Read-only access to the Web site. Users and SharePoint groups with this permission level can view items and pages, open items, and documents. Assigned to the Site name Visitors SharePoint group, by default.
Limited Access	The Limited Access permission level is designed to be combined with fine-grained permissions to give users access to a specific list, document library, item, or document, without giving them access to the entire site. However, to access a list or library, for example, a user must have permission to open the parent Web site and read shared data such as the theme and navigation bars of the Web site. The Limited Access permission level cannot be customized or deleted.