Here is an excerpt from Scott Kirsner's article, Foursquare: The 21st century's version of 'Killroy was here'?:
If you have no idea what I'm talking about, you're clearly not a Foursquare user...yet. The mobile phone software and accompanying Web site turn your daily peregrinations into a competition: every time you venture somewhere (your neighborhood Dunkin' Donuts, or the Boston Garden), you use the Foursquare app or mobile Web site to "check in," getting credit for being there. The person who has checked in the most at a particular location becomes the mayor - at least until someone else shows up more often and steals the title.
It's a way of virtually tagging your regular hang-outs. Foursquare also enables you to see which other users have checked in at a place you frequent, or leave comments and recommendations: try the mac-and-cheese at John Harvard's, even though it's pricey, suggests one Foursquare user. (Boston.com, January 28, 2010.)