What does a[href^="..."] do in CSS?

I have used CSS before and I came across the below CSS style, don't have a clue what it does.

a[href^="http:"] {
   background: url(img/keys.gif) no-repeat right top;
}
a[href^="http://mysite.com"], a[href^="http://www.mysite.com"] {
   background-image: none; padding-right:0;
}