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.

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, ...

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 ...

If you see the error message The remote session was disconnected because there are no Remote Desktop License Servers available to provide a license while connecting ...