"Being hacked" is an amorphous catch-all of a term. Typically, it means your computer has been infected with some kind of malware—a trojan or keylogger or something. And one, the Man in the Browser scam, just got a lot more sophisticated: Now it works in real time.
In the past, MitB worked by sending your transaction data—typically by creating a phony second transaction—in a raw dump to whoever was operating it, where it had to be parsed for the actual details before being sold. Now, the new Universal Man in the Browser does that work for the hacker, automatically processing the data and spitting it out on an organized webpage.
Why is this more dangerous? Because in the past, if you realized you'd just stuck your information you shouldn't have, or noticed a strange transaction, you had time to shut down the card before it was used. Now, it can be used immediately.
The best bet to avoid uMitB is to just abide by general safe practices. Watch what you click, don't download suspicious or unsecured files. But the level of sophistication of these programs is at a point where simple best practices aren't really cutting it anymore. So just do your best, and find a good malware protection service. [CSOOnline via Betabeat]