CAL stands for "Client Access License". This license grants the right for a computer to have access to the server. A CAL is not a software installation license. For example, with 10 CALs, up to 10 people at a time may have access to the server.
A User CAL will license a user to access the server software from work and home computers, Internet kiosks and other mobile devices. User CALs are typically used in companies where employees use different machines or roam from location to location. A Device CAL will license any number of users to access the server software from a single, shared device. Device CALs are typically used in companies where employees share computers and devices (perhaps on different shifts).