I can you the basic jist in a few lines. basically the cyp450 enzymes are a group of enzymes that exist in the smooth endoplasmic reticulum of your liver that primarily function to detoxify substances in your body. Basically what they do, is they chemically modify a substance so it becomes more water soluble allowing it to cross the basement membrane of the glomerulus and you urinate it out.
Now, like any enzyme, the intrinsic activity of these enzymes can be modified by certain substances/drugs. For example drugs like alcohol, rifampin, and phenobarbital induce CYP 450. What does this mean? Well, it revs up the activity of these enzymes, which in effect, will cause all other drugs in your body that are hepatically eliminated to be metabolized much quicker. The classic example exists when someone is taking rifampin for tuberculosis and is also on the birth control pill. If the physician doesn't account for this, and warn the patient, and the patient continues to have unprotected sex, rifampin will "rev up" cyp450 and cause the break down of the OCP, and walllah, your patient's pregnant.
this also works the other way around, where certain drugs such as cimetidine inhibit cyp450, and increase the half life of other drugs in your system.
hope that helps....