Commercial supports the more traditional LAN type of multiplayer. Where you either have a server which players connect to or a player acts as the server. It can (and does) function acrossed the internet - but all actions take place in one level at a time.
Pro supports an architecture like EQ. Players can connect to the server - which is actually a cluster of servers each running their own level. The servers then pass the player from one to the other as the player moves through the levels.
Each level in the multiserver format is generally refered to as a zone.
Honestly though the biggest difference is 4 for commercial 4000+ for Pro.