Various parts

What are drifting valves? I know they were located on the front of the cylinders on slide valve locomotives. What was their function and where were they located on cylinders on locomotives with other types of valve motion?
I also noticed at my recent visit to DisneyWorld that the steam locomotives there had some valves located on the bottom of their cylinders. In the PSC catalog I found a part diagram labeled lower cylinder relief valves that looked identical to what I saw at Disney. I noticed fair amounts of water, I assume it was condensation, dripping steadily from these parts.
What do they do and were these common on slide valve equipped locomotives?