What is the difference between a virus and a worm ? - Security Term

A virus is a program that replicates, i.e. it spreads from file to file on your system and from PC to PC. In addition, it may be programmed to erase or damage data.

What is a worm ? - Security Term

A worm is a self-replicating computer program. It uses a network to send copies of itself to other computers on the network and it may do so without any user intervention. Unlike a virus, it does not need to attach itself to an existing program and does not need to be transferred as part of a host to spread themselves; in other words, it doesn’t require a program to execute in order for them to spread themselves. Worms are designed to take advantage of security vulnerabilities so that they can automatically spread themselves from one computer to another. For instance when there is a vulnerability identified in any system, the programmer designs the worm to take advantage of that vulnerability and to duplicate itself so that it may spread.