Malware is a short term for malicious software. Malware is basically a way of programming which enters to a computer and disturbs it by removing data, stealing information, and many more. Malwares have many ways to get into a computer, they can get in by a corrupted e-mail or a file or even by link on the internet. Recently, malwares have become so common specially in social networking websites such as facebook and twitter. What hackers do is they put malicious sites on facebook and send them to random people, and when the users click on the link, they send them to corrupted websites. On Facebook, there was this trick going on where a friend sends you a link and tells you to a link but actually it's not your friend who sent it, so when you click you will be sent to a link that looks exactly like the Facebook login page and when you enter you username and passowrd they would steal it with all your private information with it. There was another trick but instead of a link they send you a video and when you try to watch it, the computer will give you a message saying that you need to update the flash player and when you update it, it installs a virus that will turn you computer into a zombie. A zombie computer is a computer controlled be a hacker and the hacker can use your computer as he\she wishes to without you being aware of it, which is a very dangerous thing. Twitter has been having the same problems for a long time.
This is a video that shows the most dangerous computer viruses.
These social networks are working so hard in order to stay free of Malware, recently Facebook is co-operating with a security company called Websens to protect their website, so whenever a user clicks on a link, the link will be sent to the security website for a check and then will be sent back to the user before it opens. If the website is corrupted and has malwares in it, the security website will send an alert saying the website is corrupted and it will give a reason why they think so.
This feels like a war that will never end, a war between the hackers and the security companies, hackers always creating new viruses and companies trying to protect agains those viruses.