Different lights give off different colors. Most tungsten lights (normal house lamps) give off an orange color, so I use some sort of CTO orange gel on my flash, fluorescent gives off green, so there are gels for that, and halogen and the sun tend to be bluer, so there are gels for that.
There are ways of testing, but the best way I found is to simply take a shot with the flash and the other lights in the room. Then, if you see two different colors put a matching gel on your flash to even them out. This makes indoor flash shots look a lot better, and I'm not just being nitpicky. If you are taking pics of the interior of a house and you have huge exposure extremes with the windows open and lights on you pretty much have to use a flash or two, then you will definitely notice a massive diffence with the gels on and off in terms of a nice even color all the way around.