What Barry said, but in more detail. A domain account is valid on any
machine on the network. A local account is valid only on a particular
machine. If you give a local account access to a network resource the
resource can only be accessed by that user from that machine. If you
give a Domain user access to a resource he/she can access it from
anywhere.
In addition changes to Domain users and groups can be made from any
Domain connected machine with the right software and the necessary
rights. The changes only need to be made once.