Apple attacked by Mac Defender fake antivirus software
Mac computers haven't got viruses like PCs, which has been a source of smuggery for Apple's many Jobsians.
However, that's all changed as a piece of malware called Mac Defender is tootling around the web, pretending to be antivirus software and offering to clean up your Mac. Naturally, with this being malware, it actually doesn't do your computer much good and instead, is after is your pennies.
The Mac Defender software cons Mac owners (presumably those who are particularly thick) into handing over their credit card details. What happens is that, once it's on your Mac, you'll suddenly get flooded with porn sites opening up on your computer, making you think you've got a virus (not much use if you tend to look at nothing but porn in the first place).
Then, here's the sneaky part, Mac Defender asks you to pay for a subscription and lo! The porn vanishes from your Mac! Alas, it has your credit card details and baddies can do as they wish with it.
So look out for that, and its variants called Mac Security and Mac Protector.
Mercifully, you should be able to remove these threats pretty easily by simply removing it by deleting the app from the Applications folder. Any problems there and you can open up Activity Monitor, click on Mac Defender and thunk the Quit Process button before deleting the app.