<p>Here is a snippet of CSS that I need explained:</p>

<pre class="lang-css prettyprint-override"><code>#section {
    width: 860px;
    background: url(/blah.png);
    position: absolute;
    top: 0;
    left: 50%;
    margin-left: -445px;
}
</code></pre>

<p>Ok so it's absolute positioning of an image, obviously.</p>

<ol>
<li>top is like padding from the top, right?</li>
<li>what does left 50% do?</li>
<li>why is the left margin at -445px? </li>
</ol>

<p><b>Update:</b>
width is 860px.
The actual image is 100x100 if that makes a difference??</p>

When position is absolute, top is vertical distance from the parent (probably the body tag, so 0 is the top edge of the browser window). Left 50% is distance from the left edge. The negative margin moves it back left 445px. As to why, your guess is as good as mine.