Computer infections can be broken up into 4 main categories which are explained in detail below:

Viruses

Computer Viruses are small pieces of software that attach themselves to real programs. An example would be a virus that attaches itself onto windows explorer. Each time you run the program windows explorer the virus will also be executed and perform the function that it was meant to perform. This can be several things such as damaging files on your computer, or erasing them all together. This is also the time in which the virus will try to spread itself onto other programs and even other computers!

Worms

A worm just like a virus is a small piece of software. However worms rather then relying on an external program to function, will run themselves over computer networks and security holes to spread. The worm will scan the network from it