No it is not a virus. Although many anti-virus applications pick them up.
Many people mixing things up. But there are differences between virus, trojan, worm, spyware, adware, dialers and such. Think of a virus that humans can have. You can also remove it without damaging the human. Same goes for computer viruses. You can remove them without damaging the infected file.
Adware sends you (unwanted) adds. Although everybody is saying they never asked for it, it is many times mentioned in the eula or explanation of a apllication that tells the user it does that. If people still install the application they agreed with the license terms so you could say it is in fact not unwanted at that time. It is not the creater of the application fault that people don't read before installing.
Spyware, as the names suggest, spies on you. It collects data from you and send it to the owner of the spyware. This data can be all kinds of things, like passwords, login names, where you go on the internet, what applications you use etc etc.
Ever seen a worn (little animal)? I think you did. A computer worm does the same. It tries to crawl in every direction on the net and so infecting everything it meets on it's way. If you cut a rainworm in half, both pieces can still live. Something simulair does a computer worm. It splits itself so to speak. One part remains on your system the other part leaves and tries to infect another system, where it splits itself again.
See the resembles of the names with things in real life? I think with this info you figure out yourself what the others do and how they basicly work.
Just don't forget: There is NO application that can detect ALL harmfull things. That's why you need to use multiple. Personally I recommend to use these 4:
Avast
Ad-Aware
Spybot s&d
Hijackthis
