Understanding weapon system in Neverwinter Nights

I just got a free copy of the old game Neverwinter Nights Diamond Edition so I decided to give it a shot. For a person who's not played old RPGs, this was a head hurting experience, because i was unable to understand stuff. But I slowly learned bit by bit. Still one basic thing I'm unable to figure out is weapon system. I somehow found what is 1d4 and similar stuff. But what about attack speed? How do I know that? And second what is Base Critical Threat and does the damage type and size of the weapon affect attacking in anyway, because it all felt the same to me, or maybe I'm wrong. Nevertheless, if someone can explain this or even put up a link which would help me out, would be greatly appreciated. Thanks!