What is a Client Access License (CAL)?

If your company's workstations are networked, you will utilise a network server and the workstations on the network will access that server(s) software to perform certain functions such as file and print sharing. In order to legally access this server software, a client access license or CAL may be required. A CAL is not a software product; rather it is a license that gives a user the right to access the services of the server.

Find out about the different types of CALs

Software Assurance

Microsoft Software Assurance is a comprehensive maintenance offering that helps you get the most out of your software investment. It combines the latest software with phone support available 24 hours a day, partner services, training, and IT tools that help customers deploy, manage, and migrate software. With Software Assurance, you can increase worker productivity, accelerate organizational performance, and quickly realize a return on your software investment

More info- http://www.microsoft.com/licensing/sa/default.mspx