Top 3 malicious codes

Virus
A virus is a program or a piece of a code that is loaded onto your computer without you knowing and automatically runs. Even simple viruses can easily duplicate it self which will take most of your computers memory which will cause it to slow down until it comes to a complete halt. An even more dangerous virus is one that is capable of sending itself across networks which will then harm more computers and will spread.

Worm
A worm is a program that replicates itself over a computer network and usually performs actions, such as using up the computer's resources and possibly shutting the system down.

Trojan horse
A trojan horse is a destructive program that covers up as a beginning application unlike viruses, trojan horses do not replicate themselves but they can be just as destructive. One of the most insidious types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer.