A worm is a program designed to replicate.
The worm program may perform any variety of additional tasks as well.
Worms are programs that run independently and travel from computer to computer across network connections.
Worms may have portions of themselves running on many different computers.
Worms don't change other programs, although they may carry other code that does.
The characteristics of a worm is listed here:
Worms are autonomous agents capable of propagating themselves without the use of another program or intervention or action by a user.
Worms are found primarily on computers that are capable of multitasking and are connected by a network.
Most worms disrupt services and create system management problems.
Some worms scan for passwords and other loopholes and then send the information back to the attacker.