Spyware
Software that sends information about your Web surfing habits to its Web site. Often quickly installed in your computer in combination with a free download you purposely selected from the Web, spyware transmits information in the background as you move around the Web.

Spyware programs can also change the home page in the browser to a particular Web site or just keep popping up their ads all the time. Nevertheless, once you detect spyware, it can be eliminated, though sometimes with much difficulty.

Dialers
Programs that disconnect your telephone connection to the Internet and initiate another call to a premium-rate number (mostly international) running your phone bill through the roof

• Virus
  Software used to infect a computer. After the virus code is written, it is buried within an existing program. Once that program is executed, the virus code is activated and attaches copies of itself to other programs in the system. Infected programs copy the virus to other programs.

• Spyware
  Software that sends information about your Web surfing habits to its Web site. Often quickly installed in your computer in combination with a free download you purposely selected from the Web, spyware transmits information in the background as you move around the Web.  Spyware programs can also change the home page in the browser to a particular Web site or just keep popping up their ads all the time. Nevertheless, once you detect spyware, it can be eliminated, though sometimes with much difficulty.

• Dialers
  Software that sends information about your Web surfing habits to its Web site. Often quickly installed in your computer in combination with a free download you purposely selected from the Web, spyware transmits information in the background as you move around the Web.
• Worm
  A destructive program that replicates itself throughout disk and memory, using up the computer's resources and eventually taking the system down.
• Logic Bomb
  A program routine that destroys data when certain conditions are met; for example, it may reformat the hard disk or insert random bits into data files on a certain date or if a particular employee record is missing from the employee database. Most viruses are logic bombs because they deliver their payload after a specific latency or when a trigger event occurs.
• Macro Virus
  A virus that is written in a macro language and placed within a document. Viruses have to be "run" in order to do things. When the document is opened and the macro is executed, commands in the macro language do the destruction or the prank.
• Word Macro Virus
  A virus written into a macro that is stored in a Word document or template. There are more than 30 different kinds of this virus. When the document is opened, the macro is executed and the virus does its damage. It also attaches itself to the Normal template in Word so that subsequent documents are saved with the virus. Questionable documents should always be opened with macros turned off.
• Trojan Horse
  A program that appears legitimate, but performs some illicit activity when it is run. It may be used to locate password information or make the system more vulnerable to future entry or simply destroy programs or data on the hard disk. A Trojan horse is similar to a virus, except that it does not replicate itself. It stays in the computer doing its damage or allowing somebody from a remote site to take control of the computer. Trojans often sneak in attached to a free game or other utility.

Rootkits
Hackers install what is called a root kit that writes over certain system files with the hacker's version of the program and eventually the system. Once the "root kit" is installed onto the system, the hackers will kill the original processes to start their own allowing them complete control/access of your computer/ network.

Viruses vs. Spyware
This is probably the question asked most often. What is the difference between viruses and spyware?

Viruses; they infect other programs and data. It behaves like a virus and it spreads throughout your computer looking for even more programs to infect. A computer worm also infects but you might consider it a trained virus because it spreads without user intervention.

Spyware; usually refers to any non-viral malware. Examples of this are Trojans that pretend to be what they are not (like games or screensavers or utilities or even anti-malware). Bottom-line is that spyware spies on you. It gathers personal info and sends it back to criminals. Spyware gathers passwords, logons, etc. In addition to Trojans, there are key loggers and rootkits infiltrating your defenses and often going undetected.