Basically there are three options that you can use in licensing your SQL 2005 server for either the Enterprise, Standard or Workgroup editions. Each of the three licensing modes is listed below with brief descriptions of each.
With this licensing mode each processor whether they are physical or virtual that the SQL server uses for SQL must have a license. It does not require a Device or User Client access license (CAL) each of which will be explained below.
Device Client Access License (CAL)
This licensing mode requires a license for each of the Network Operating Systems (NOS) where the NOS is running a SQL instance rather it is a named instance or the default instance. It also requires you to have a CAL for each client machine that accesses the SQL server.
User Client Access License (CAL)
With this licensing mode a license is needed for each NOS that is running a SQL instance rather it is a named instance or the default instance in addition to a CAL for each user that accesses the SQL server.