I have searched google, but nothing worth while comes up, and I've searched the forums, but I haven't found an answer to this question: How does an antivirus program actually work?

If someone knows of a thread or tutorial/lecture that has the answer, please post it(I like to learn/figure things out on my own most times. However, this time I think I need a hint or two, he he). Otherwise, if one of you has the answer, that would be greatly appreciated too. Thanks for your time.