title: What is a pixel (px) in CSS?
body: <p>I am reading an ASP.NET book and it refers to CSS files and talks about pixels. But I never understood it from a resolution, layout, etc. point of view. For example, what does the following mean in CSS file definition? </p>
  
  <pre><code>#header
  {
      padding: 0px;
      margin: 0px;
  }
  </code></pre>