Parasite's all programs that invade YOUR computer with out your permission to be called Parasites, They can be broken down by categories by the way they invade your computer or what they do when they get there.
Adware The ABC clothing company hires a Adware company to place their ad on your computer, it sits idle until you type into your internet browser a keyword like "shoes" and up pops a window advertising the ABC clothing company, the Adware company may have a long keyword list, so pop ups keep coming. 

Spyware Some Adware may contain Spyware, but normally Spyware is software that silently leaks information to a third party and needs a internet connection, the third party may do many things with the information they get from you, including selling your information like a mailing list would be sold.

Browser Hijackers Homepage hijackers change the page that appears when the browser starts up. If all of a sudden your home page of google.com changes to a low-rent search portals full of pay-per-click advertising and often with no actual search results....you've been hijacked. Search hijackers change the search engine used when search queries typed in at the address bar and/or the search sidebar.

Dialers Software that makes outgoing phone calls to premium-rate numbers.Generally the dialer (or ‘dialer’, in US English) is ‘providing access’ to commercial content, typically porn. A modem (analogue or ISDN) must be connected to the computer for this to work; cable or DSL connections cannot, on their own, make outgoing calls. A stealth dialer makes its calls without any prompting from the user, and typically turns the modem speakers off so the calls are not audible. The charges for the calls arrive after the fact on the victim’s phone bill, which may go unnoticed.

Cookies Most cookie's (a file placed on your computer by most websites) are safe and actually speed up your browsing of frequent websites. However, many of the anti-parasite programs also detect and remove cookies from certain advertising networks, as ‘tracking cookies’ or even ‘spyware cookies’.

The Scammers Anti-parasite programs which themselves install parasites, here is a small list of the most popular one's that popup and ask you for approval to install them Mynetprotector, Scumware remover, SpySpotter, Malware, Ucontrol, Spyban, terminexor, SpyAssult, VirtualBounser, WareOut, SpyBlast, AdProtector, SpywareNuker, Spyware Avenger, MypcTuneUp, Jimmysurf, Warnet, KillAllSpyware, SpywareHelp, SpyWiper, Ad-Eliminator

Virus A virus is a program that propagates itself by infecting other programs on the same computer. Viruses can do serious damage, such as erasing your files or your whole disk, or they may just do silly/annoying things like pop up a window that says "Ha ha you are infected!" True viruses cannot spread to a new computer without human assistance, such as if you trade files with a friend and give him an infected file (such as on a floppy or by an email attachment).

Trojan Virus This is a very general term, referring to programs that appear desirable, but actually contain something harmful. The harmful contents could be something simple, for example you may download what looks like a free game, but when you run it, it erases every file in that directory. The trojan's contents could also be a virus or worm, which then spread the damage.

Worm Like a virus, a worm is also a program that propagates itself. Unlike a virus, however, a worm can spread itself automatically over the network from one computer to the next. Worms are not clever or evil, they just take advantage of automatic file sending and receiving features found on many computers.