Selling client-access licenses (CALs) is big business for Microsoft. In fact, it's believed to be a $1 billion a year business for the Redmondians.
CAL licensing is a regular source of confusion, and something about which I and no doubt other Microsoft watchers get lots of questions.
Any user or device that accesses Microsoft server software directly or indirectly legally requires at least one CAL, Microsoft officials say. There are a number of different types of CALs: Device CALs, User CALs, Base and Additive CALs, Premium CALs, Enterprise CALs and more. Different Microsoft products often require different kinds of CALs. And "external users" -- those who aren't employees or onsite contractors -- also may require CAL coverage, too.
Update: As a few readers noted, if you have to ask whether you are supposed to pay for a CAL, the Microsoft answer is far more often than not, "yes." But there are still questions worth asking to make sure you don't overpay for CALs.