I've been wearing a backpack to carry my clothes, etc when I commute. I generally don't carry a whole lot of stuff, but occasionally I do bring a laptop.

I'm tired of my back getting so sweaty and the overall discomfort of wearing a backpack, so I'd like to try panniers. They don't need to be gigantic. In fact, I'd prefer to keep them on the small-to-medium size so they don't create too much drag. And I'd like them to be waterproof. I commute on a mountain bike sometimes and on a road bike sometimes, so I'd like to be able to easily move the panniers from one to the other if that's possible. Now for the questions...

Do I need some kind of rack on the back of my bike to use panniers? Do different panniers require different kinds of racks or can I just use a "universal" one? Are there certain features I should look for or avoid? And finally, if you recommend a certain brand/model I would like to know about it. Thanks in advance.