title: What is a database index?
body: <p>I've heard them talked about since I started working in tech about 18 months ago.  I know that they potentially improve performance, and they seem to be column specific -- ("We index the User table on the date_of_birth column").</p>
  
  <p>Just looking for a quick overview of what exactly they are, what they are used for, and how they work.</p>