I've been out of the licensing arena for a while but I want to say you buy the SQL server and Cals to use it on clients... CALS can be used as per user (so me using my laptop, desktop and mobile device is one CAL) or per device so that a workstation is licensed and it doesn't matter if John, Mary and Sue all use it.
I think larger enterprises have site level licenses that may be econimacal depending on the implentation but those are very expensive compared to cals for small businesses.
I think there used to be per processor type licensing that went away with multi-core chips...
Of course all this stuff changes about once a year so to what degree I am off in 5 years, who knows.
The easiest thing to do would be to call one of the big three and talk to a Volume Licensing expert... By big three I mean the big three computer sellers (CDW, Inight, PC Connections). You might post back here with a general licensing question because you are really talking about client access (Access front end is not relevant information and some people tend to ignore questions about it in the SQL forums).