Virus - A computer virus is a program usually hidden within another simple program. It produces copies of itself and inserts them into other programs or files, in turn destroying the data and performing other malicious actions. Computer viruses are never naturally occurring; they are always man-made.
Worms - Worms are very similar to viruses in the manner that they are computer programs that replicate copies of themselves (usually to other computer systems via network connections). Unlike viruses, worms exist as separate entities; they do not attach themselves to other files or programs.
Torjan Horses - A Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses into the system. The Trojan program does not attach itself to the files like a virus nor replicate itself like a worm but it does provide unauthorized access to user’s computer. They are mostly spread through internet downloads and online gaming programs. They mostly affect the targeted computers.
