What is Virus, Worm and Torjan?

Virus:
Virus needs some media to attack other machine. Media like Floopy disk or USB, its more vulnerable.

Worm:
Generally this does not need any media, to attack another machine. If any folder is shared, worm spreads from one machine to another machine and it attacks.

Torjans:
These are more dangerous in nature, these are generally used by attackers to create backdoor.

Backdoor? - Having attacker the control of victims Windows shell (like command prompt). Using which attacker can whatever he wants.