The kidneys serve two major functions: they keep the salt content of the blood constant, and they filter waste out of the bloodstream. So, the main components of urine are (salt) water and waste products. The major waste product from cells in the body is ammonia, and the major waste product from blood is a broken form of heme called bilirubin. In the liver, each of these is converted into a less hazardous form: ammonia is converted to urea, and bilirubin is degraded to urobilins. Salt, water, and urea are all colorless, but urobilins (which come from degraded pigments) are yellow. So, if you drink a lot, your urine will be more dilute and clearer, and if you get dehydrated, your urine will contain less water and be darker yellow. And that is why pee is yellow.

I've found that your urine can also be shades of red. Not from blood in the urine as is sometimes the case when one has a serious problem, but from eating something with extremely concentrated coloring - like beets. But this only occurred when my wife baked the beets like a potato instead of boiling them. Boiling obviously leeches out a lot of the color, and, no doubt, some of the nutrition. If you like beets, try baking them until done, then soaking overnight in whatever flavoring you usually use - delicious. I suspect your urine will be pink.

yeah, it's pretty much because of the urobilins and stercobilins (produced partly from recycled blood cells and hemoglobin) that make up this yellowish color, as cojoda described. :)