What is Viruses, Malware, and Spyware

Malware, or malicious software, is a class of software designed specifically to wreak havoc on a computer—your computer. Malware includes such nasty entities as viruses, Trojan horses, worms, and spyware. If you’re experiencing frequent crashing, nonsensical error messages, pop-up advertisements (other than when surfing the Web), or slower-than-normal performance, the culprit may be one of the following types of malware (as opposed to a feature authored by Microsoft):

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. Unlike worms, defined next, viruses can’t infect other computers without assistance from people. One particular type of virus, a Trojan horse, spreads itself by masquerading as a benign application (as opposed to infecting an otherwise valid file), such as a screensaver or even, ironically, a virus removal tool.

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. Worms can replicate themselves like ordinary viruses, but do not spread by infecting programs or documents. A classic example is the W32.Blaster.Worm, which exploited a bug in Windows XP, 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 purpose 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 unsuspecting users, but most often to serve as a conduit for pop-up advertisements (known as adware).

Aside from the ethical implications, spyware can be particularly troublesome because it’s so often very poorly written, and as a result, ends up causing error messages, performance slowdowns, and seemingly random crashing. Plus, it uses your computer’s CPU cycles and Internet connection bandwidth to accomplish its goals, leaving fewer resources available for the applications you actually want to use.

Now, it’s often difficult to tell one type of malicious program from another, and in some ways, it doesn’t matter. But if you understand how these programs work—how they get into your computer, and what they do once they’ve taken root—you can eliminate them and keep them from coming back.

Source of Information : OReilly Windows Vista Annoyances Tips Secrets and Hacks