First, a reminder on what a Foursquare "friend" is: the only people connected to you as Foursquare friends are those people with whom you have a bilateral agreement to be friends. In other words, someone can only become your Foursquare friend if you actively approve their request to be friends, or if you invite them and they approve your friend request. Both parties have to agree to become friends. 
 
Your Foursquare friends can see most of your personal profile and location information, while the wider Foursquare community (and sometimes your Facebook and Twitter communities) can see a more limited set of information, depending on a robust set of choices we give you during the check-in process and on your User Settings page (foursquare.com/settings or by clicking ‘Settings’ on the Foursquare website). The check in process and User Settings give you a lot of control, so that it is easy to share your location information exclusively with your Foursquare friends (and even among this close group of Foursquare friends, you can check in privately, in which case your Foursquare friends will not see that you’ve logged a check-in at all but you'll see it in your private check-in history). 

‪Note: Private check-ins unlock badges but will not disclose the location or venue tied to the badge. Private check-ins will not unlock a Mayorship, nor they will count towards the Mayor tally.
 
There is also the opportunity to publish some of your Foursquare activity to your Twitter and/or Facebook accounts. Of course, when you publish Foursquare check-ins, Mayor unlocks, or Badge unlock notices to Twitter and Facebook, such information will be viewable by your communities on those services. Please see here for more information on what happens when you choose to connect your Foursquare account to your Twitter and/or Facebook accounts, and how you can control information sharing across these services.