Well, first off, aMule is a fork of xMule which is a fork of lMule which is a port of eMule. Thus, all 3 apps share a certain feature-set. lMule was the original port of eMule from Windows to Linux, but it haven't been updated in quite some time and thus is falling behind the other three. It would be safe to consider lMule obsolete. aMule is a more recent fork of xMule, which had become the de facto replacement for lMule, and thus, has more in common with xMule than with lMule, since both are being actively developed, with the result that old code tends to be removed. Regarding specific features in the two, I can't say much since I have little experience with xMule of recent times. In the future, the differences will become more apparent, however at the moment, I am hard pressed to name a "killer" feature that aMule has and xMule hasn't and visa-versa.
As for the reason for the forks, those can be attributed to mainly two things: Clashes of personalities and conflicting interests in the plans for the future of the client.
