A worm is very similar to a VIRUS in that it is also replicating software. The difference between a virus and a worm is that a worm replicates itself independent of a carrier object. It does not need to attach itself to a file or a document to spread or execute. In other words, if you have a worm on a disk, as soon as you put the disk in your computer, the worm begins. A virus would only start when you opened the infected file on the disk.
Program that infests a network environment and exponentially multiplies itself thereby using larger and larger amounts of memory and disk space until it brings the system to a halt. There is no hard line between viruses and worms; in general, if the spreading entity is a self-sufficient program, it will be called a worm, whereas if it embeds itself inside other programs or boot code, it will be called a virus.