Servant Channels are channels that are used by other channels to perform some delegated functionality. For instance, a Webmail channel might delegate to an Address Book servant channel to provide a user experience around looking up an email address. Once the user has found and selected the email address, the servant channel returns control to the full-fledged channel to continue the user experience. Several channels delegate to the Groups Manager Servant Channel to accomplish selection of groups and the Permissions Manager Servant Channel to accomplish assignment or editing of permissions.
How do servant channels differ from stand alone channels?
Stand alone channels are "real" channels that can be actually placed in a user's layout. They're the primary channels. Servant channels come into being in an adhoc way when called by other channels, and go away again when that primary channel is done with them. They are not persisted in the layout.