"A general term for malicious software that is designed to take control of a computer without the consent of the user. Adware is one type of spyware - computer programs in which commercial advertisements are automatically shown to the user without their consent." (Wikipedia)
How does it get there?
Sometimes when signing up to a website or downloading software the software agreement points out that you’re installing a little more than you thought. These sites rely on the fact that most people don't read software agreements. Sometimes free software really is too good to be true, and bundled alongside them are programs that will bombard your PC with advertisements, or track your browsing habits online for research purposes.
What does it do?
Typically this software will flash up adverts at regular intervals, or will re-direct web pages to particular sites, so that the user has little control over what is seen in the internet browser. They can seriously affect the usability of the PC.
What can be done about it?
Most good anti-virus software packages also have anti spyware as well. This was not always the case, but effectively this sort of software has been grouped with a virus because it can seriously impede your use of the computer. With this software installed the software will not be able to run.
However, if the software licence was agreed to by the user then it may be more difficult to uninstall the software. It may be possible to uninstall from the control panel as with any program, but sometimes it is necessary to go on the web to search for a solution. There is usually some identifier which you can use to put in the search engine.