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When it comes to brewing, there are two basic types of barley. One is 2-row, and the other is 6-row. All this really refers to is the morphology of the grain "head". With 2-row barley, there are two rows of large mealy grains; with 6-row, there are six rows (big suprise) of somewhat smaller grains. The former is what is used when malting pale malt. It's more malty than 6-row, has a higher husk-to-grain ratio (important when lautering), lower in protein, and host of other characteristics. 6-row has a grainier flavor but has a lot of excess enzymes that can be used to convert the starches in adjuncts--which is why it is favored by domestic macrobrewers. Many brewers are saying that domestic 6-row is catching up to 2-row in quality, however.
Pale ale malt is pretty much the same thing as pale malt--I think the only real difference is what the maltsters call it. Marris Otter is a variety of barley that is prized by British brewers, and homebrewers for that matter, because of its excellent malty flavor. It is, more or less, pale malt. There's also other varieties of barley like Optic or Halcyon that are also malted to create premium grade pale malts.
Also, different maltsters from different countries have varying grades of pale malt. Cheapest and most readily available for homebrewers is domestic 2-row/pale malt from maltsters like Briess, Cargill, or Schreier. Don't let anybody try to tell you that you're doing the wrong thing if you use one of these malts--they're excellent in their own right. I've used both Briess and Schreier to great succes--they're just not quite as malty and rich-tasting as imported pale malts. British maltsters include Muntons, Crisp, or Fawcett; Belgian include Dingeman's; German include Weyermann, Weissheimer, and Durst. As you might expect, you'll pay a premium for some of these imported malts, often 50-75% more than for an equivalent amount of domestic pale malt. However, you really do get what you pay for, but again, that's not to kick dirt at the domestic malts.
In short, I recommend trying out a bag of domestic 2-row or domestic pale malt (Briess actually has both, for some reason). If you're new to AG, it'll help you get a feel for the process without breaking the bank on the more expensive imported malts. Hope that helps!
