What are the different roles?

There are four roles: Administrator, Editor, Author, and Contributor.

Here is what they can do:

Administrator

The Admin can do everything. Complete power over posts, comments, settings, theme choice, import, users - the whole shebang. Nothing is off-limits, including deleting the entire blog.

Editor

They can publish any posts, edit any posts, edit any published posts, edit any pages, moderate comments, manage categories, manage links and upload files. They can also delete any posts and any pages. They can read, edit and delete private posts.

Author

Is able to edit their posts, publish their posts and upload files. And they can delete their posts too.

Contributor

They can edit their posts but cannot publish.

Advice
Be careful what roles you give people, especially if you want them to be an Editor or joint Administrator - Editors can delete content, and Administrators can remove users, including you. Promoting someone is easy - removing a role is much harder.