title: What is the purpose of GL_COLOR_BUFFER_BIT and GL_DEPTH_BUFFER_BIT?
body: <p>As an OpenGL beginner I would like to know what do they do and why these are required. For instance in the call</p>
  
  <pre><code> glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  </code></pre>