Where does "Places" get all its information?
Places is part of Google Maps (and was introduced with Maps 4.4 about a year ago), you should be able to see and search for all the same businesses and attractions in the main Google Maps app, Places just puts a nicer interface on it.

Much of the data came from what used to be called Google Local. Businesses can add themselves to Local/Maps/Places and update their records themselves, here http://www.google.com/local/add/promo Google Maps has had a "Add your business to Google Maps" link on the page pretty much forever, that links to http://www.google.com/local/add?hl=en&amp;gl=us

Also just like other local business discovery services like Foursquare, users themselves can add places into Places, check into them and rate them through the app or http://www.google.com/hotpot

See also the announcement page from when "Place Pages" were first added to the web version of Google Maps back in 2009 which has some more details http://googleblog.blogspot.com/2009/09/place-pages-for-google-maps-there-are.html including that you can add them through community edits or Google Map Maker.