You put a light bulb into a socket, turn it on, and you have light. Light only comes in one form, on or off. Or does it?
Believe it or not, a light bulb is putting out more than just lighting, it’s also putting out color. Unless you live in a black-lighted gothic room, it’s probably putting out a shade of white light. If you’ve ever painted a house you know that there are more shades of white available than anything else. So the question is, what shade of white is your house being lit with (and why does it matter)?

To see this represented visually, take a look here or above. White light will "match colors produced by a black-body radiator at certain temperatures in kelvin." I’m not entirely sure what that means, but looking at the graphic, notice how color temps to the left are more red/yellow, and color temps to the right are more blue. If you are at a computer (sorry to my minions of cell/pda readers) you probably have a color temperature setting on your monitor. The generally accepted "whitest" color is around 6500k. Try playing around on your monitor setting the color temperature higher and lower, and you should see the difference as shown in that chart.
So what does this all mean for your light bulbs? The cheapy regular incandescents most of us have in our house have a color temperature rating of less than 3000k, a far cry from that holy grail of 6500k. This makes the lighting tend to be very yellowish. However, usually ignorance is bliss. If every light looks yellowish, you probably can’t tell. If you have flourescent bulbs in your kitchen though you might be able to see a difference. Most flourescents tend to be closer to 6500k than most incandescents. In our house the kitchen has always had a very nice white look, but the neighboring dining area is very yellow. Having these two areas right next to each other made the difference very obvious.
If you’re curious and want to see the difference, I recommend checking out some of the "natural" or "daylight" bulbs sold in stores. They usually come in two varieties. Flourescent bulbs usually have a high lumen output (how bright they are) but most that I found were the spiral variety (bad for most of our fixtures). The other type are incandescent bulbs (old school kind) that are made with special glass that filters out the yellow (leaving the light "whiter"). You can browse some different bulbs here where I just bought some (the selection in most stores is lacking).
Make sure that when you are looking you can find out the lumen rating. Some of the daylight bulbs I’ve seen in stores are only rated at 500-600 lumens, but a regular incandescent is closer to 800 lumens. Lots of the daylight bulbs don’t list a color temperature rating, but if they’re advertised as daylight or natural, they should be closer to 6500k.