Prophecies, Factions, and Nightfall are all "standalone" games that require nothing else. Each takes place on a separate continent, with its own storyline, quests, skills, items, etc. If you have more than one associated with a single login account, they sorta "link up" and form a game world that you can travel freely around in as you please. Having all three gives you access to everything, which is obviously preferable.
If you're going to choose just one for now, Prophecies and Nightfall are generally regarded to be the best options for new players as they have the best learning curve. Nightfall would be my personal recommendation.
Eye of the North is an expansion that requires one of the three above, and mostly adds content appropriate for veterans of the game.
I recommend you read
this for much more in-depth info before you make a purchase.
