2012 is likely "to go down in the history books as the Year of the Super PAC," says Heather Michon at Salon. These independent, private groups are spending a fortune on ads — often far more than the candidates themselves. And super PACs aren't just about money. Some have evolved into full-fledged "shadow campaigns," putting out messages that their favored candidates can't control. How did this happen? Here, a brief guide:

What is a super PAC?
A political action committee, or PAC, is an independent group formed to raise and spend money to elect and defeat candidates. PACs, which have been around since 1944, can give just $5,000 per election to a candidate and $15,000 to a party. Super PACs, on the other hand, are new political creatures, authorized to raise and spend as much as they want on campaign ads.

And super PACs don't violate election laws?
Not anymore. In 2010, the U.S. Supreme Court said in its controversial Citizens United ruling that independent third parties have a constitutional right to raise and spend as much as they choose on political ads. That, the court said, is a matter of freedom of speech. The court did stipulate, however, that super PACs and their deep-pocketed donors must not coordinate their activities with any campaign. Otherwise, the court warned, campaign finance laws would be meaningless. Still, many super PACs, including those supporting Mitt Romney and Newt Gingrich, are run by people long associated with the candidates they support.

Just how much are super PACs spending?
A lot. So far, independent expenditure committees have spent more than $38 million on the Republican candidates, which is three times more than the 2008 candidates spent themselves over the entire 2008 primary season. Advertising by the actual candidates is down by 40 percent this cycle, but thanks to an astronomical 1,600 percent increase in spending by outside groups there are more campaign-oriented spots on TV and radio than there were four years ago. When you see or hear a campaign ad this year, the odds are nearly 50-50 that it's the handiwork of a super PAC. A telling example: The pro-Romney Restore Our Future super PAC fueled his pre-primary surge in Florida with a bombardment of 12,768 television commercials in the state, compared to just 210 by Gingrich. 

Are super PACs really "shadow campaigns"?
Many campaign insiders complain that they are. Some super PACs have moved way beyond ads, and are armed with phone banks, pollsters, direct mail, and surrogate spinmeisters.

Is this good news for candidates?
Not necessarily. When super PACs send mail or speakers to make direct contact with voters, they can muddle messages that campaigns have carefully crafted to address sensitive issues. "It would be much better for the super PACs to just focus on running ads and not try to get into the ground game," Jesse Benton, Ron Paul's campaign manager, tells Politico. The often vicious super PAC attack ads could be a double-edged sword, say the editors of the Palm Beach Post. "What happens if a super PAC runs an ad that backfires? The candidate could pay the price for a message he or she didn’t even condone."

Sources: Open Secrets, Palm Beach Post, Political Wire, Politico (2), Salon