A computer virus is software that attaches itself to another program to survive and reproduce and can only run if the infected program is running. This can tie up resources such as disk space and memory, causing problems on any computer.
An email virus is the latest type of virus which is transported through email messages and usually attaches itself to all contacts on the victims email address book.
A worm is similar to a virus and infiltrates security holes on your PC. Once a security hole is found, the worm will attempt to replicate itself from computer to computer.