They initially posed as an Adobe Flash Player installer, then have acquired the capability to disrupt the automatic updating of XProtect, the operating system's built-in anti-malware application.
At the beginning, the user was responsible for downloading and running the malware, but lately even that step has been removed as newer variants have begun being dropped on targeted systems via an exploit of unpatched Java vulnerabilities.
According to F-Secure researchers, the latest variant of the malware - Flashback.K - is being distributed to Mac users through the misuse of one such vulnerability (CVE-2012-0507) that has already been patched in the Windows version of Java.
Unfortunately, Mac users haven't received a patch for that particular vulnerability since Apple hasn't yet ported it to Java for Macs. In addition to all that, there are rumors that an exploit for another unpatched Java flaw is being offered for sale on online forums.
The researchers advise Mac users to disable their Java client for the time being in order to avoid infection.
UPDATE: Apple released a patch for the Java flaw mentioned in this article.