Malware = Malicious Software
The term "malware," derived from "malicious software," refers to any software specifically designed to harm a computer or the software it's running. Because malware can potentially steal sensitive information like credit card numbers or passwords from your computer, send fake emails from your email account, and more, it is often referred to as "crimeware."
Malware can be installed on a computer, with or without your knowledge, in a number of ways—usually when you visit a contaminated website or download seemingly innocent software. It can also be designed to infect your internet browser(s) as well via the use of extensions and add-ons.
Some cybercriminals attempt to use advertising to distribute malware. Possible vectors of attack include malicious code hidden within an ad creative (such as a swf file), embedded on a webpage, or within software downloads.
Ad networks vary in their responses to malvertising incidents. At Google, the Anti-Malware Team studies malware distribution and works closely with the security community to identify crimeware on the web and share that information more broadly. This stems directly from Google's security philosophy: we believe that if we all work together to identify threats and stamp them out, we can make the web a safer place for everyone.