OS X Mountain Lion: What is malware?
If a warning message tells you that something you have downloaded from the Internet is “malware,” you should exercise caution. The safest action is to put the item in the Trash, and then empty the Trash.
Malware is an abbreviated term for malicious software. Malware includes viruses, worms, trojan horses, and other types of software that can damage your system or violate your privacy. Malware can be installed on your computer when you download content or app from the Internet, either from email or websites.
Certain instances of malware are merely harmless or annoying. More often, its intent is to take control of your computer to collect personal information, host illegal content, send spam email, or cause harm to other systems on the network. Personal information that’s collected often includes credit card, banking accounts, social security numbers, or other identifying information leading to identity theft and financial loss.
Avoid opening items downloaded from websites and email messages unless you are certain that they come from a legitimate, trusted source. If you are uncertain about the source of a downloaded item, it is best to delete the item. You can always download it again later, after you have made certain that the item is not malware.
This article provides information on safe download malware protection: