They give clients "permission" to access servers. If a client accesses a server and there's no CAL for that client, you're in violation of your licensing agreement. They're kept on paper basically.

Microsoft announced this week that it is making a change to its Core CAL (Client Access License) bundle-- something of interest to its business customers who purchase enterprise products. Microsoft ...

As of December 1, Microsoft is changing the way it prices the "user" option when purchasing client-access licenses (CALs), which will result in higher prices for some customers. With the User CAL, ...