What is VIRUS/worm/Throjan Horse/logic bomb?

VIRUS :
Viruses are malicious software programs that have been designed by other computer users to cause destruction and havoc on a computer and spread themselves to other computers where they can repeat the process.
A virus is an independent program which reproduces itself. It may attach to other programs, it may create copies of itself. It may damage or corrupt data, change data, or degrade the performance of your system by utilizing resources such as memory or disk space.

WORM:
Worms are programs which reproduce by copying themselves over and over, system to system, using up all resources and thus sometimes slowing the systems. They are self contained and use the networks to spread, they are very much similar to viruses(use files to spread).

THROJAN HORSE :
A program or utility that falsely appears to be a useful program or utility, such as a screen saver. However, once installed, it performs a function in the background such as allowing other users to have access to your computer or sending information from your computer to other computers.

LOGIC BOMB :
Code which will trigger a particular form of 'attack' when a designated condition is met. For instance, a logic bomb could delete all files on Dec. 5th. Unlike a virus, a logic bomb does not make copies of itself.
It is unintentional, It can be an error in the logic of a program that results in the destruction of the data. It can be the result of a simple corrupt file.