Party

A party, in the D&D world, is defined as a group of people who are setting out to achieve one common goal. Whether that goal be to slay the black dragon at the bottom of the dungeon, or find huge piles of loot and gold in the abandoned cathedral, their main goal remains the same - work together to defeat common enemies and do whatever it is they have set out to do in the first place.

Online Party
If you are playing NWN2 in an online server, you may join a party to play on the server with them. You can help eachother complete quests, slay monsters, gather treasure, and in general, it makes most aspects of the game easier.

Offline Party
If you are offline, playing the Main Campaign or an offline module, your party will consist of NPCs, who will act in the place of a human ally. They aren't as intelligent as a real human would be, but they can get the job done. NPCs offline are called companions. You will unlock more companions as the game progress (in the Main Campaign), who can then travel with you and aid you in combat or conversations.