Types of computer viruses - how to classify a virus

There are many types of computer viruses - trojan virus, worm, malware, rootkit, downloader virus, spyware, brontox virus. Most of us called all of these as 'virus' generally because we dont know the exact name of the malicious program that infects our computer, so we called it a virus. To classify a virus, you need to check on it's infection style and and the infection effect on your computer. Does the virus makes your computer slow? Does your monitor filled with several pop-out advertisements? Or does your folders being hidden and the fake folders are being created over and over again most of the time? These virus infection signs shows up usually when your anti virus software database is not up to date, even you have anti virus software fully operational and works well.