Malware: refers to a large variety of software which all has one thing in common: it is unwanted software which someone else wants to run on your computer. This software “infects” your computer, making it behave in a way which you do not approve of. Malware can include:
Viruses: which are computer programs that can copy itself and infect a computer without permission or knowledge of the user.
Worms: which are self-replicating computer program. It uses a network to send copies of itself to other nodes (computer terminals on the network) and it may do so without any user intervention. Unlike a Virus, it does not need to attach itself to an existing program. Worms almost always cause harm to the network, if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer. are usually destructive in some way - these can delete files, wreck the operating system, etc.
Trojan Horses or Trojans: which are software which purports to do a certain type of action, but in fact, performs another. The trojan is not harmful by itself but it usually opens a backdoor in the infected machine for an attacker.
modern viruses and worms, which give control over your computer to an attacker - they can then use your computer to send spam or launch attacks on other computers
Spyware: which monitors what you do on your computer - this can lead to credit card or identity theft
Adware: which shows you unwanted advertising - this typically shows up as pop-up ads or an unwanted homepage