Well socket A is basically a previous generation for the Athlon XP, Durons, Semprons (although Semprons also have Socket 754 now)
The Socket A is also known as Socket 462 because the CPU has 462 pins, Same thing applies to the Socket 754, 939 and 940
Socket 754 = No dual channel support (first thing that pops into my mind, not sure if theres another main differentiating difference)
Socket 939 = Supports everything and will continue to be upgraded by AMD
Socket 940 = Requires ECC and Registered RAM ONLY and again will be phased out just like Socket 754
Of course more pins = more bandwidth = more performance.
So each one has it's own features.
Higher the # the faster......well kinda...you gotta look at the seperate features and whatnot, but generally speaking it's true.
Like I said, more pins = more bandwidth for data to flow through.
That's just a quick run down I'm sure someone else can give you more detail.