Transfering characters to campaigns?

Okay so I got my Guild wars up and running finally. I have the trilogy now and waiting to get EoTN until I am positive I am gonna play for an extended period of time. My question is when creating a character, you're able to choose from prophecies classes, Factions, and Nightfall. Are those special classes to Factions and Nightfall limited to those campaigns only? Or can I play say an Assassin in Prophecies?

You can only make assassin/ritualist in Factions so you have to play there until you get off the starter island, then you can go wherever you want.

Like majoho said you can only make an assassin or ritualist in factions for example but you can take these characters to Prophecies and Nightfall once you get just a little further into the game.

when you create a char from any campaign they can play in any campaign by crossing over to it from a port city

Lions Arch.. (Proph)...Kiang Center (Factions)..Consulate Docks (Nightfall)

To get to EOTN within the same area in all campaigns above.

Each has its own quest to get to a particular campaign or EOTN

You can travel to other campaigns through certain quests. look through wiki.guildwars.com for general info about the game ;-). hope you enjoy gw

Be careful while browsing the wiki it's full of spoilers.

Just as a quick reference, to travel from prophecies you need to get to Lion's Arch, to travel from factions you have to get to Kaineng Center and to travel from nightfall you need to get to the Consulate Docks.

There is no way to take a campaign-specific profession (Assassin, Ritualist, Dervish, Paragon) to Prophecies and experience the whole campaign. The game assumes that a foreign character is picking up the storyline at the point where a native character would have reached the port area for that campaign. You can go to earlier areas (except for pre-searing in Prophecies due to no time-travel) and do the missions.

Once you reach a port area, the wiki has a list of quests for campaign travel.
http://wiki.guildwars.com/wiki/Campaign_Travel