What is a DOCTYPE? Which one do I use?

According to HTML standards, each HTML document begins with a DOCTYPE declaration that specifies which version of HTML the document uses. Originally, the DOCTYPE declaration was used only by SGML-based tools like HTML validators, which needed to determine which version of HTML a document used (or claimed to use).  

Today, many browsers use the document's DOCTYPE declaration to determine whether to use a stricter, more standards-oriented layout mode, or to use a "quirks" layout mode that attempts to emulate older, buggy browsers.