I'm frequently asked what the difference is between a virus, worm, spyware, malware, keyloggers, root kits, and rogue programs.  Here's a quick summary for the curious:

Malware

Malware is a catch-all term that includes computer viruses, worms, trojans, most rootkits, spyware, dishonest adware, crimeware, rogue programs, and other malicious and unwanted software.  Many of these threats will NOT be detected by most anti-virus and anti-spyware programs.  Often these programs will also detect and disable the most common programs which could possibly detect and remove them.

Viruses, Worms, Trojans, & Hijackers

Viruses are programs that have the ability to replicate themselves and spread when executed.  Back in the old days they caused people to lose data by formatting their hard drives, but today they're more of a nuisance than anything else.  Viruses frequently these days install spyware, adware, and root kits onto your computer. 

Worms are similar except once they're running they have the ability to spread via security flaws to other computers on their own.

Trojans are named after the Trojan Horse.  They're designed to collect your passwords by masquerading as legitimate programs.

Hijackers are usually found in the form of browser hijacking.  They change your homepage and default search providers, and frequently redirect you to fake sites for collecting passwords and credit card information.

Spyware, Adware, Key Loggers, and Root Kits

Spyware is just that, software that spys on what you are doing with your computer and reports it back to the author(s).  Unfortunately there are lots of programs out that are perfectly legal, but due to the lack of privacy practices on the part of their authors, they're classified as spyware.  One of the most common of these is "MyWebSearch."

Adware is a program that causes popup ads on your computer.  You generally know for sure these programs are installed when they pop up ads on the homepage for Google.  Frequently the ads they pop up are pornographic.

Many people are tricked into installing Spyware and Adware onto their computers because they're bundled with other software such as small games.

Keyloggers are programs that not only keep track of what you're doing, but what you're typing also.  This enables someone to hijack your passwords to everything from your e-mail to your online banking.  Because these programs are running on your computer, it makes no difference whether you are communicating through a secure connection or not.

Root Kits are the worst of the bunch.... they usually include spyware, frequently include adware & keyloggers, and further allow someone to take control of your computer remotely.  They also frequently can not be removed from within windows... not even in safe mode.

Rogue Programs

Rogue programs are usually the result of a pop-up during websurfing telling you that spyware or a virus has been detected on your computer.  Once you install them, they will detect all kinds of spyware, viruses, and other malware... even sometimes other Rogue Programs, but will not allow you to remove these "threats" (which don't really exist much of the time.)  Most are simply people trying to hijack your credit card information with very annoying programs that will not go away until you "register" them, and include no way to uninstall them once they are installed.

Solutions

As the saying goes, the best defense is a good offense.  I recommend keeping a reputable virus scanner (such as AVG, available in a free version for home use) and a Spyware protection program (Such as Windows Defender, which is also freeware and automatically part of any Vista or Windows 7 installation - But you have to turn it on to make it work.) installed on your system. 

I also recommend keeping around and using regularly a tool such as MalwareBytes' Anti-Malware around to detect and clean the peskiest of adware, spyware, rogue software, and root kits from your system.