Official Google Webmaster Central Blog: Malware reviews via Webmaster Tools
What is malware?
Malware is a software that tracks your moves online and feeds that information back to marketing groups so that they can send you targeted ads.
Malware can be downloaded with you even knowing by playing online games, downloading software that includes it and they don't inform you, etc. that install the software that tracks your every move.
Most of the malware installed will show you random pop-up ads but some peoples' computers slow down or even crash. The malicious software will use peoples' personal information and abuse it.
Google will now show you results in the search engine result page (SERP) that will warn you if a website uses malware. Some site owners don't even realize that something on their website has malware on it, so they are letting webmaters know through the Google webmaster tools if they have malicious software etc. on their site so they they can remove it.
I always love to see the user coming first when it comes to websites and search engine optimization and this is another great way that Google is helping the webmaster optimize the website, and pay attention to the user.