Adware

Adware is a type of software that sometimes comes with free downloads. Adware displays advertizements on your computer, and some monitor your computer use (including websites visited) as well, so they can displays targeted ads based on your usage.

Adware is considered malware by most, and as such, is sometimes guarded against by anti-virus software. More specifically, it is often classified as a Potentially unwanted program. There are also programs which are more focused on non-virus malware, like Ad-Aware and Spybot which will try to block and/or remove this unwanted software.