White Balance

Color temperature is a term sometimes used to describe white balance.
White balance is basically how white objects are tinted- "warm" blue, or "cool" red, which is opposite of what we typically think about those colors. Sunlight typically leans more towards red on the black body spectrum, and tungsten lighting is typically shifted more towards blue (it can also be yellow, depending on the light). Flourescent lighting is normally some sort of green.

All this really means is if theoretically you take a picture of someone wearing a white shirt, outside on a sunny day it will be slightly tinted blue, inside it will be tinted slightly red or yellow, and under flourescent light slightly green. This applies to everything in the shot, not just perfectly white objects.

To correct white balance before shooting, many cameras have adjustable white balance settings (example, mine has tungsten, overcast, sunny, flourescent, etc, and a custom setting). With film, because you don't have these options, and also with digital if you don't want to use all the buttons, a colored filter can be added to the lense or a gel can be stretched over the flash.

To shoot inside you would use a blue gel/filter, outside you would use a red/yellow filter, and in flourescent lighting a magenta filter. These take some playing around with, but you can easily change the white balance to whatever you want.
Of coruse thanks to Photoshop, we can also quickly adjust white balance levels after taking a shot.