Malware: "Malicious Software"
Describes generally all "hostile" software.  

Adware: Application that displays advertising banners.  These annoying programs run in the background without your knowledge. They may cause pop up ads, or follow your browsing habits.  Many hijack your browser’s homepage and prevent you from changing it back.  You also may notice extra links added to keywords on a normal web page that direct you towards Adware sites.

Spyware: Transmits information back to a third party without notifying the user.  These programs can read credit card numbers you enter into web page forms and send them back to the programmer. You could also fall prey to programs that monitor what websites you visit and send this data back to marketers who will be able to build up a full picture of you and your interests without your permission. 

Most unwanted Malware will “piggy-back” onto your system along with something you wanted to install. A popular freeware program will be offered for download.  If you install it, other programs are installed too.  Some Malware might try to sneak past you with a pop up offer.  If you answer yes, you have given permission to install almost anything into your browser. This is how most browser hijacks occur.  Don’t click YES or OK to any pop up offers.

There are some excellent programs available to combat the rising tide of Malware.  I recommend Lavasoft’s Ad-Aware SE Personal Edition and Safer-Networking’s Spybot.  Remember to keep your definitions up to date and scan your system often.  Also, be sure to keep Microsoft’s latest security patches up-to-date: Windows Update Site