Info on AMD Semptron & Athlon 64 needed

I need some info on the AMD semptron processor. I understand that this processor is available in two flavours, Socket A & Socket 754 . Please tell me-

1. Whats the difference between the two?.

2. Which is the newer/better standard?.

Also, I see that the Athlon 64 is available in two flavours also. The 754 pin & the 939 pin. Please tell me

1. Whats the difference between the two?.

2. Which is the newer/better standard?.

Thanks.