My AntiVirus software keeps talking about downloading "virus definitions". I don't understand what that means, or why I would want to do it. Can you explain?

When dealing with biological viruses, you want to have an antibody, which will either protect you from being infected or, if you've already been infected, it will start killing off the virus. But there isn't one antibody that works for all viruses, which is why scientists are always working to find new ways to combat new viruses. Computer viruses are very similar. A virus definition is like an antibody. It's really just information about a particular virus, which will help the virus software fight the virus, or protect you from infection. And just like each biological virus requires a different antibody, each computer virus requires a virus definition. Do you want to download the virus definition files whenever your computer recommends that you do so? Absolutely!