Malvertising
A method used by malicious users to help distribute their malware by advertising downloads to their products through online advertising companies or by hijacking a websites to display their ads. For example, a visitor may see an advertisement for a free anti-virus or security utility that in reality is a malware related product.
Although larger advertising networks are good at detecting and preventing these ads from being displayed, these ads can sneak through any advertising network, especially smaller networks and have the capability of being distributed to a wide audience.
Also see: Malware, Security definitions, Spyware