Socket 754: is single channel only, for single channel AMD Athlon 64 processors. It will also eventually accomodate the plain Athlon XP which will essentially be like the duron. This is and will continue to be the "bargain computer" socket.
Socket 940: the dual channel socket for AMD's opteron processors. As far as I know, this socket will continue on in the server market and is in the desktop only temporarily.
Socket 939: the soon to be introduced redesign of socket 940. It has the voltage pins and memory bus pins shuffled around to make it easier to go with a motherboard design with fewer layers, which means the prices will drop. ie: this will have the registered and/or ecc memory pins removed and voltage will be supplied by more of them (if I've read correctly, manufacturers need to worry less about crosstalk with this design). The one pin they left out will have a blank in the actual socket so no one will stick an Opteron in there and screw things up.
Spelling and grammar are beautiful things to learn, by the way

Sam