I know next to nothing about S&W semiautos,can someone tell me the difference between the M&P, Sigma and 39/5900 series and any others I might have missed?

Too long to answer, You should wikipedia it.

You got totally different firing mechanisms, Safeties, alloys, metals verus plastics...on and on.
It would probably be best to just google each one and see their characteristics.
Kinda like asking what's the difference in a Corvette, toyota, and a Saab car in general. That would be quite a posting

I suggest you Google S&W semi-auto handguns and do a little research yourself. Too much to try to explain here. First gen. have two digit model no's.,second gen. have three digits,third gen. have four digits. Some are all steel, some have alloy frames. Then you get into polymer guns.....etc.