If you visited Forbes.com between November 28th and December 1st of last year, then a virus may have secretly been downloaded onto your computer. Regular Forbes readers will already be familiar with the Thought of the Day splash page that you see whenever you visit the site. That's what hackers targeted.
Hackers exploited a zero-day exploit in Adobe Flash. A zero-day exploit takes advantage of a bug that no one knows about. This increases the amount of time that it takes for security researchers to develop a fix.
They combined the Flash exploit with another zero-day bug in an outdated version of Internet Explorer.
If you use Firefox or Chrome and visited during those dates, then you weren't vulnerable to this attack.