Macs have a pretty solid reputation as being 'virus-free.' For the most part, Mac users don't have to install anti-virus software and worry the way PC users do. Recently however, a malware program pretending to be a anti-virus software for OSX has become an issue. So much of an issue, that Apple itself has posted a response, removal tips, and are releasing an update to the OS.
Malware is software that is, most times, mistakenly user downloaded. A bad link from a trusted site, such as Facebook or Google, could prompt a message that will ask you to install "x" software or program. If you're not sure what it is you're downloading, the best bet is to stay clear. (Same goes for Windows users.)
Phishing and malware attacks can be cleverly disguised, so always be sure of where you're getting your software from online. If you get an email from Amazon, for example, make sure the return address is from Amazon. You don't want to install anything, or give important personal information to anyone trying to trick you.
For details and support with 'MacDefender' (or MacProtector/MacSecurity,) you can see Apple's official statement here.
And as always, PC or Mac, if you're having virus issues you just can't seem to get rid of, feel free to let us take a look. We'd love to help.