Okay, so I'm just wondering what the difference between shampoo mediums is. I know there are three types: traditional (liquid or gel in a bottle/tub/whatever), dry (powder?), and bar.
Do these all work the same or is there a difference between them? And if so, which do you prefer?

Dry shampoo is more to tide you over between washes. It soaks up excess oil/grease and you comb it out. You can't really get by just dry shampooing but it's good for a day or so.

Bar and gel shampoos depend on what type you get as to what they do, but they're the ones that actually wash your hair.

iamtheparty is correct on all points. The difference between traditional and bar soaps isn't so much about the medium, but what's inside it. Read the labels carefully and avoid ingredients that include alcohols and acids, as they are much more likely (chemically speaking) to strip your color out.

If you're looking for a very gentle shampoo, look for the JASON line (health food stores mostly) but if you're in a crunch, there are lots of kids shampoos that will work well too.

I would imagine that bar shampoos are less conditioning (conditioning products don't lend themselves to being in a solid bar form).