Spyware is a catchall term to describe computer programs that are designed to stealthily install themselves on people's computers -- often when the users attempt to download seemingly legitimate programs. The most benign spyware programs -- also called "adware" -- simply serve up a barrage of pop-up messages, while the most intrusive can track online movements, steal passwords and hijack sensitive data.

Although some of the most intrusive uses of spyware are already illegal, many of the programs aren't covered under current law, a disparity that prompted the current clamor on Capitol Hill. After spyware began making headlines last year, lawmakers introduced no fewer than four bills to tackle the problem.