Viruses, Malware, and Spyware

Malware, or malicious software, is a class of software specifically designed to wreak havoc on a computer. Malware includes such nasty entities as viruses, Trojan horses, worms, and spyware.

Viruses:

A virus is a program or piece of code that "infects" other software by embedding a copy of itself in one or more executable files. When the software runs, so does the embedded virus, thus propagating the "infection." Viruses can replicate themselves, and some (known as polymorphic viruses) can even change their virus signatures each time to avoid detection by antivirus software.

Worms:

A worm is a special type of virus that can infect a computer without any help from its user, typically through a network or Internet connection. A common example is the W32.Blaster.Worm, which exploited a bug in Windows (eventually fixed as part of update #824146), causing it to restart repeatedly or simply seize up.

Spyware and adware:

Spyware is a little different than the aforementioned viruses and worms, in that its intent is not necessarily to hobble a computer or destroy data, but rather something much more insidious. Spyware is designed to install itself transparently on your system, spy on you, and then send the data it collects back to an Internet server. This is sometimes done to collect information about you, but most often to serve as a conduit for pop-up advertisements (known as adware).