From Microsoft:

With Microsoft Security Essentials, you get high-quality protection against viruses and spyware, including Trojans, worms and other malicious software. Security Essentials is easy to install and easy to use. Updates and upgrades are automatic, so there's no need to worry about having the latest protection. It's easy to tell if you're protected - when the Security Essentials icon is green, your status is good. It's as simple as that.
When you're busy using your PC, you don't want to be bothered by needless alerts. Security Essentials runs quietly in the background, only alerting you if there's something you need to do. And it doesn't use a lot of system resources, so it won't get in the way of your work or fun.