Instagram is a social photo sharing service. You post photos and people that "follow" you can see them. It's really that simple. Once you download the app for your device (iPhone app, Android app, and Windows app) and sign up for an account, you are all set to share. Instagram is a free service and the associated apps are also free. Instagram (owned by Facebook) has talked about inserted ads into your list of photos (not into your actual photos, however), but so far that hasn't happened yet.