The term "malware" is short for malicious software and is usually used as a catch-all term to refer to any software that causes damage to a computer, server, or computer network. Some of the most common types of malware are listed below.
Self-replicating malware requiring a host file that depends on human action to spread it.
Self-contained malware, needing no host file, that spreads automatically through networks.
An apparently useful and innocent application containing a hidden malicious program.
A program that secretly monitors your online activity and sends the data back to the programmer.
The actual type of malware, however, is not usually significant, as most modern malware is what's referred to "blended threats," that is, taking bits from several types and combining them.
While not in itself malware, a botnet is a giant network of "zombie computers" that can be used to carry out the instructions of the botnet owner. Botnets are often rented to scammers and hackers and can be used for everything from sending spam to attacking foreign governments.
Is your computer part of a botnet? The chances are pretty good that it might be. One security researcher recently put the number of "botted" computers as high as 24 million.
Trend Micro's Simply Security blog has an excellent article detailing the current state of botnets.
TopTenReviews provides independent reviews of the best-known antivirus software products available for consumer use. In addition, AntivirusWare's Avoiding Viruses and More provides several easy tips on keeping malware off your computer in the first place.