For the Wikipedia definition of Malware, please see http://en.wikipedia.org/wiki/Malware

Malware is code designed to exploit and infect the person visiting a website. Generally speaking this is accomplished by either compromised FTP accounts or via exploited code on your website (SQL injections, file upload, etc). The code embedded in your site will tend to be one of the two examples
(NOTE: This is not a complete listing, only examples):

Hidden Iframe – Loads a hidden frame that will infect the user
Obfuscated Javascript – Will directly exploit the users browser

A few good resources to use to help ensure that your sites are not infected would be:

1)    Stopbadware.org – http://www.stopbadware.org - Offers a database of bad sites as well as resources to help fix issues
2)    McAfee SiteAdvisor - http://www.siteadvisor.com/webmasters/index.html
3)    Google SafeBrowsing – https://transparencyreport.google.com 

Snowtech does receive reports from various organizations in regards to MALWARE and has a system designed to relay these reports to you.  Please note that it is your responsibility to ensure that your site is not listed as malicious otherwise our automated system will continue to open tickets reporting that your site is marked as malicious.