The difference between single- and multiplayer is that in multi, there are more than one character player created. If you've played IWD, then you'll know how it works.
Basically, when you start or load a game, then you have to choose which characters are going to be in the party, and that each time. You can throw out characters, import them or others on this screen (this is, before the actual game starts/continues), and basically play with whichever party you want to,
but that is the only difference.
The PC, in singleplayer games merely the first character of the selections, is in mulitplayer also the first character (on the char selection screen, it's the one on the top left). The stronghold quests, the romances, everything depends on the first character.
Heck, you can even create a mutliplayer game, and copy the save into your singleplayer savegame directory, and play it as such, the only drawbacks will be, that your own characters can be thrown out, but not taken back into the party once that happens, and the less character interaction if you choose to do this.
And, it seems, to answer your question, I once again managed to explain a lot more than asked for
