Guild Wars Question

Does anyone know if I need to get the original guild wars to play factions and to have factions to play nightfalls?
If I just get nightfalls can I play all the characters?

Each Guild Wars Game is a seperate world, and you are not forced to buy the original Guild Wars.
If you get nightfall (which rocks, the Wintersday events were awesome!), you can access all the original GW classes plus the nightfall classes (Dervish and Paragon), but not the factions classes (Assassin and Ritualist).

but you will only have access to the Nightfall campaign and Nightfall skills.

(of course, many skills are non-game specific, and some even duplicate each other)