Whenever you create a user on a blog WordPress requires that you assign a role for them on the blog to which you are adding them. This can be one of the following:
Administrator: Has full rights to do anything on the blog including and up to deleting the blog. Note the limitations on an administrator’s ability to create new user accounts as discussed in here.
Editor: Has full editorial responsibilites including the ability to approve posts and moderate comments. They cannot change the design or settings on a blog.
Author: Can write, publish and edit their own blogposts but not those of others. They can add media such as pictures to blogposts.
Contributor: They can write and edit their own blogposts but their posts require the approval of an Editor or Administrator before they can be published. Once published the contributor may not edit their post anymore. Contributors cannot add media to their posts for several reasons (click here to read an article about this).
Important note: unless further security measures are added to your WordPress Multisite, contributors are able to view all comments including unmoderated comments on their posts. Creative Blogs uses a plugin called Easy Blogging as well as a number of other measures to mitigate this and thus children are able to blog without being able to see any unmoderated content.
Subscriber: Used rarely in schools, the subscriber role simply gives somebody membership of a blog. It is used to give users a login to private blogs or to give a user the ability to comment on a blog where commenting is restricted to members only.
Roles are applied on a blog by blog basis: i.e. a user could be an administrator on one blog and an editor on another. For a full list of the different capabilities of each user see this list on WordPress.org.