Folding forum is a great resources for folding bikes (well duh).

But basically, there are two sorts, the folding bike, which includes the Tikit, most of the Dahons and Bromptons, where you can fold it up real small and take it with you on the train or into the office.

And then there are the packable bikes, like the NWT and the Pocket Rocket (which I have), which fit into a suitcase so you can take them on vacation. Hell, you can even use the suitcase as a trailer if you want. They do fold, but it's an ungainly, clunky fold that would be awkward in a train.

I admit I can't see the advantage of commuting on a packable bike, although I love riding it.