title: What is a private key in Android?
body: <p>I read <a href="http://developer.android.com/tools/publishing/app-signing.html" rel="noreferrer">this article</a> about signing your Android applications. I used Eclipse to export my Android application, had to create a keystore (which succeeded) and a private key with an alias (which also succeeded). So I know that I have to sign the application with a private key.</p>
  
  <p>However, the article does not make clear anything on the following questions:</p>
  
  <ul>
  <li>What is a private key?</li>
  <li>Should each application you make have another key or should they share the same private key?</li>
  <li>What is an alias?</li>
  <li>Why do the applications have a validity lifetime?</li>
  </ul>