Is the large pan under the engine for the transmission or differential or are they combined?

what color was the fluid? it was more than likely the transmission fluid, and you do NOT want to drive it without that. In most cars the transmission oil is red, but you will want to make sure you add the correct stuff back in- the grades of transmission oil are not universal between brands. Your owners manual will tell you how much transmission fluid it holds- put it in and then let the car run for a little bit and then check it again to make sure the level is right- it should be checked warm, not cold. If you don't know how to do this, have a professional do it because you don't want to do it wrong or overfill it.