Contributors are people you have allowed to write entries on your blog. There are three different types of contributors:
Authors can write, edit and delete their own entries on the owner's blog, but not edit other contributor's entries.
Moderators can write, edit and delete their own entries on the owner's blog, but also edit and delete other entries.
Administrators have the same privileges as the blog owner, so they can write, edit and delete their own and other entries as well as creating groups, and changing the blog's appearance - everything that a blog owner can do.
Example: In a research group blog you may have a number of different contributors. You could give research students 'Author' permission so that they could write about their work. You could give fellow department staff moderator permission so that they can not only write their own entries but also edit other entries. You may also want to give someone in the group 'Administrator' rights so they can manage the blog in your absence.