Software forced onto the unsuspecting masses to combat Viruses. Usually written by ex-virus writters who needed a steady income.
Differs from viruses in intent, and cost. Viruses are usually free, whereas anti-viruses are not. Anti-virus software also tends to have a flashier interface.
Both software types will severely impact your ability to do anything productive, both will occasionally greet the users with unwanted popups, add web tool bars, provide inaccurate (but sponsered) search results, and keep your local IT shop in business.
"The new server anti-virus installed succesfully on the first try. Unfortunatly we couldn't access anything anymore, but we were protected, and that was the important part.