It is a malware. A worm is an autonomous agent capable of propagating itself without the use of another program or any action by a person. In simple words it's a self-replicating program. It's very similar to a computer virus, but not the same. Worm is self-contained and is not a part of another program. That means that worms don't need another program to propagate itself. They are engineered to exploit the file transmission capabilities found on many computers.
The first know worm was the Morris.
Maybe few years before, it was just a self-replicating code, in the past few years worms and viruses became serious pests: spreading around the world in a matter of hours with the capacity to carry highly damaging payloads.
What does the worm do?
A worm may be designed to do various things. It can delete files on a host system or send documents via email (spam which is also infected with the copy of worm), cloak web sites addresses or even a payload. It can slow down you computer system's running, and can harm your system. A common payload is for a worm to install a backdoor