Can anyone explain the following wire set up?

At a single pole switch I have:

1 - 2 wire (black and white - probably the source)
1 - 3 wire (black, red and white - going to the ceiling box)

At the ceiling box I have:

1 - 2 wire (black and white)
1 - 3 wire (black, red and white - from single pole switch)

Where does the 2 wire at the ceiling box go to? What is it for?

I'm trying to install a ceiling fan (w/ light) where just a light used
to be. Seems as though when I get everything hooked up right (I
think) - then all other outlets in room are effected by what the fan
(not the light) is doing on the ceiling fan.

Thanks, Darb