Every time I ask myself the question which type of CAL do I require? It always seems to take some thought in order to comprehend the difference.... Especially when you only ask the question every couple of years.

But I found this picture which helps visualize the difference between Per User and Per Device CALs and the source website may be helpful too:

Answering the question of whether you can buy a mix of user and device CALs ==> http://emerset.com/product-licensing/articles/server-licensing/per-user-or-per-device