title: How do iphone apps know about other apps?
body: <p>I recently downloaded an app (Overkill) and they have an in game "virtual currency"  called "overkill medals" or "OM". You can use the in-app purchase system to buy more OM, or you can earn it, through their service called "Tapjoy." </p>
  
  <p>Tapjoy (inside the Overkill app) brings up a menu with eligible apps, and a link to the iTunes store to download it and requires you to "download and open" the app in order to earn your OM. </p>
  
  <p>How does this app know that I have downloaded and opened the app, in order to award me the OM?</p>
  
  <p><strong>Edit:</strong> It also seems to know when I had the app already</p>