Questions about expansions

So how exactly do these work? I notice that some require you to own a previous GW game (EotN) And some don't (Nightfall) so what exactly do these do? Do they add to the game? Are they a whole new game? Is there any point to buying all of them or are they upgraded versions of the previous game? Any answers would be helpful, I intend on getting Guild Wars + EotN expansion from Newegg but they also have Factions for cheap and I want to understand what the expansions do.

Thanks ahead of time!

Eye of the North is the only expansion: Prophecies, Factions, and Nightfall are independent campaigns. You may play any of the campaigns separately or together, but you must own one of the campaigns to play EotN.

So say I get my character to 20 in Prophecies, can I use the same character on Factions? Also, can I use the Assassin class in Prophecies or Nightfall?

I guess the real question is: is there any point to own more than one version?

Yes, that is correct, you may play in the other campaigns or expansions if you link them all to one account.

If you want to create an assassin or ritualist, that character has to start in Factions. Similarly a dervish or paragon would have to start in Nightfall.

Certain pvp areas such as Alliance Battles, Jade Quarry, and Fort Aspenwood require access to Factions. Nightfall has heroes. Prophecies has pre-searing Ascalon.

If you want more than one of the standalone games, Guild Wars Trilogy might be a good option for you.

prophecies (tyria), factions (cantha), nightfall (elona) are 3 separate worlds. they are however interconnected, and characters can travel between them. eotn opens up a new area that takes place in the same tyria as prophecies, however you can travel there from any of the 3 campaigns.

the main difference between eotn, and the others, is that you cannot create a new character there. and for that reason, no new additional character slots will be given to you, as well no new classes will be available.