What is a DOCTYPE and what does it do

<body> <center>Sorry you do not have frames enabled. Here is the link of the tutorial:<br> <a href="http://www.divitodesign.com/blog/2007/05/what%e2%80%99s-a-doctype-and-what-does-it-do/">http://www.divitodesign.com/blog/2007/05/what%e2%80%99s-a-doctype-and-what-does-it-do/</a></center> </body>