What is Malware?

Malware is a broad term used to describe any number of different types of infections your computer can get. Malware is any kind of malicious software, including viruses, trojans, spyware, adware, and worms.  While some malware can simply be an annoyance, other forms of malware can severely damage your computer causing programs to crash or even stopping them from running. There are also forms of malware intended to steal personal information or to prevent an internet site from providing its services to customers and members (Distributed Denial of Service attack or DDoS).
Trojans (aka Trojan Horses) are some of the most prevalent forms of malware in my experience. Trojans are so named because they often come disguised as a legitimate program such as a game hack or even an antivirus software!
Spyware is a form of malware that can track and collect a users personal information. Often spyware can be hidden in something legitimate such a music download. Adware is quite a bit like spyware, except that its main purpose is to present various advertisements to a PC user.
A worm is a self-replicating program that can send copies of itself to other computer users over a network. Most worms were designed to simply spread themselves and their payloads(i.e. the worm's "cargo").
A virus is a malicious program that can infect computers when its "host" program is run. It can infect other programs and spread to other computers by being passed on in an infected program. Computer viruses are very much similar to a human virus.
These are broad definitions of a few types of common malware. There are other types of malware in existence, which I will go over at a later date.
Remember, one of the first steps to prevention is with knowledge