There's an entire chapter in the Server Reference Manual about that, pasted from there:
• Named CAL (an identified user on a server) - Access is based on user identityand valid for all documents on the server, that is any number of concurrentsessions from one user on one machine at a time is allowed.• Document CAL (an identified user within a given document) - Just asabove, the access is based on user identity, but the CAL is valid only for onedocument. If the same user connects to two documents using this licensingmethod, he will hence consume two Document CALs.• Session CAL - Each Session CAL allows one user to access one QlikViewdocument, that is to have one session at a time. Anonymous users areallowed, no identification of the client user is necessary.• Usage CAL - Each Usage CAL gives the right to initiate one session (singledocument) per running 28-day period. The session may last a maximum ofone hour. Any activity after the first hour has expired will count as a newsession (albeit without visible interruption). No identification of the clientuser is necessary.
In regards to Desktop and Server, both are QlikView products, and in order to develop documents with Desktop you will need a Named User CAL. Server has its own license, and there are several server editions available, depending on your deployment.
Anyway, contact your partner who knos your system and needs better.