adware 
The somewhat derogatory name given to ad-serving software that you install unknowingly when you download free software, such as games, utilities, or file-swapping programs. In an effort to create revenue from a free service, some software developers are agreeing to let third-party software--in this case, software that displays ads--piggyback on the freeware you download. The software installs along with the requested program. Notification of the additional software is usually buried in a lengthy licensing agreement that you must accept in order to get the program you requested. The adware continues to run in the background even if you close the requested program. If you uninstall the original program, that doesn't remove the adware. A similar type of third-party software is spyware.