When you choose to create a new HTML document via CTRL+N or File > New..., you get the following New Document dialog:To start with a basic blank HTML page, you should select HTML for the Page Type and "<none>" for the Layout. But what about the DocType? The DOCTYPE is used to specify the "flavor" of HTML you want to use.
It used to be pretty important to understand the different DOCTYPEs, but with the introduction of HTML5, there is an easy way out, which we highly recommend as it is both simpler and backward-compatible. DOCTYPEs differ, so just go with the new HTML5 DOCTYPE as it's easier to remember.
It is possible that you are working on a development team or project that has standardized around one of the other DOCTYPEs. If so, you may want to stick with that one. However, unless otherwise instructed by a manager or site administrator, you should choose the HTML5 DOCTYPE.