Computer virus or virus is small piece of a software or application that attaches itself to a program. It can be a self generated or engineered by someone and launched. Once it is attached to a program, it runs every time you open that program or application. For example, if it is attached to an Excel Sheet, it will run and duplicate itself to other applications, every time you open the Excel Sheet. Sometimes its duplication speed is too fast to contain and at time it develops to spread even without opening of the file or application it attaches itself to.
Computer E-mail Virus
This type of virus is attached to an email, either as an attachment or on rare occasions even without attachments. It often has attractive names such as I LOVE YOU that can tempt users to open the attachments. Once opened the virus can quickly email itself to all email address in your address book. It can also open without you clicking on it, especially if your email system does not have a scanning of email facility.
A worm is a piece of a software or application that use gaps in network security to copy itself to a system. It travels very fast from a system to another system, without even opening the files which replicates itself to. It is more difficult to contain and even more damaging to network security system. It is particularly ruthless in scanning the entire system files to another system in a corrupt mode and causes huge losses in seconds.
A Trojan horse is a program i.e. a game, which claims to be safe, but once you open it, it causes damage to your system as much as any other virus or worm. It is particularly quick in erasing data from your hard-disk. However, it does not replicate itself to other systems and networks.