Clickjacking

Suppose your site lives at www.kittens.com. Unless you have implemented protective measures, a hacker could build their own site with a very similar URL, and include your site in an iframe.

www.k1tt3ns.com/vacuum-revenge
Watch This Kitty Get Revenge On A Vacuum Cleaner In The Cutest Way Possible
A cat being scared of a vacuum cleaner
An iFramed Website
<html>
  <head>
    <style>
      body {
        position: relative;
        margin: 0;
      }

      iframe {
        border: none;
        position: absolute;
        width: 100%;
        height: 100%;
      }
    </style>
  </head>
  <body>
    <iframe src="www.kittens.com/vacuum-revenge">
    </iframe>
  </body>
</html>