Foursquare, a year-old social networking startup, is seemingly everywhere these days — the South By Southwest music and technology festival, Starbucks, even the TV show "Gossip Girl." What is Foursquare, and what's all the fuss about? (Watch a FourSquare tutorial)

What's Foursquare's main feature?
Foursquare allows you to share your location with friends. So, when you walk into a bar or restaurant or mall, you "check-in" (Foursquare's term) on your cellphone or mobile device, and your friends will see where you are on a Foursquare map.

Is that it?
It's also a game, of sorts. You earn points and "badges" for checking-in frequently, or at a certain time. If you raise enough points you become "mayor" of a certain area.

How many people use it?
It just surpassed 500,000 registered users, and had its biggest day ever March 11, with more than 275,000 "check-ins."

Is it safe to let everyone know where you are?
Only your Foursquare friends are able to access information about your whereabouts, and only if you choose to let them know. Its founder says it is more "tightly curated" than most social networks.

But isn't Facebook planning something like this?
Yes — as are Twitter, Yelp, and other larger, more established sites. Foursquare's "very existence depends on surviving this battle" for a major piece of the "checking-in" market, says The Business Insider's Jay Yarrow.

How big is the company?
With just 16 employees, not very. But it raised $1.35 million in capital last September, and more venture capitalists are lining up.

Why is Foursquare in the news now?
It just turned a year old, and it's making a big push for wider adoption at the South By Southwest (SXSW) festival in Austin. So-called geo-location services are being heavily promoted at SXSW this year, with Foursquare, Gowalla, Loopt, and others vying for the same kind of boost that SXSW gave Twitter in 2007.

What sets Foursquare apart?
It is being taken seriously by businesses. Starbucks is the latest and most prominent company to sign a deal with Foursquare — you can earn a "Barista badge" by checking in five times at the coffee retailer. Foursquare says it now has 1.4 million venues logged in its system, with 1,200 businesses offering special deals to people who check-in via Foursquare. 

What's in it for the businesses?
Starbucks is testing Foursquare as part of its customer-loyalty program, and other companies are also using it to reward frequent visitors. Foursquare is rolling out a free analytics program that will give participating businesses detailed information on who is checking-in to their locations, and give them the ability to communicate with the visitors.

What's in it for Foursquare users?
Connecting with friends, the fun of earning badges, and various perks from the places where they check in. Foursquare also gives you access to your check-in history, giving you a snapshot of where you go, who you meet there, and what you're spending money on.

Was Foursquare really mentioned on "Gossip Girl"?
Sort of. On this week's show, a character's cellphone shows that another character "just checked in to the Algonquin Hotel." The check-in wasn't necessarily "through Foursquare proper," says The Business Insider's Yarrow, "but obviously, this is a shout out for the company that popularized the check-in."

Sources: USA Today, GigaOm, New York Times, Business Insider, Lost Remote, Wikipedia, TechCrunch, Mashable