As always, it's difficult to do more than simply elaborate slightly on sneff's excellent node.
I myself avoid the term "kaffir lime" because of the pejorative connotations of the word kaffir. I tend to call them "wild limes", and often refer to the leaves as just "lime leaves", which probably isn't strictly speaking correct, but is less offensive.
Like other aromatics, for example lemongrass, lime leaves are best bought fresh if at all possible. Avoid dried lime leaves if you can; the flavour is rather flabby and dim, whereas the fresh ones are fabulously fragrant.
Suddenly located fresh lime leaves (most likely in Chinatown)? Afraid the motherlode won't last forever? Don't despair. Lime leaves freeze well. The flavour, it is true, is compromised slightly, so I usually use two frozen leaves in place of one fresh.