The first line on the top that starts <!DOCTYPE... is to let the browser know that you know what the hell you're doing. You may think that you don't actually know what you're doing yet, but it's important to stick this in. If you don't, browsers will switch into 'quirks mode' and act in a very peculiar way. Don't worry about this just yet, you can learn more about 'document types' in the HTML Advanced Guide if you really want to. For the moment, just remember to shove this line at the top of your web pages and you're laughin'.
To get back to the point, <html> is the opening tag that kicks things off and tells the browser that everything between that and the </html> closing tag is an HTML document. The stuff between <body> and </body> is the main content of the document that will appear in the browser window. Closing tags
The </body> and </html> close their respective tags. ALL HTML tags should be closed. Although older versions of HTML lazily allowed some tags not to be closed, latest standards require all tags to be closed. This is a good habit to get into anyway.
Not all tags have closing tags like this (<html></html>) some tags, which do not wrap around content will close themselves. The line-break tag for example, looks like this : <br />. We will come across these examples later. All you need to remember is that all tags must be closed and most (those with content between them) are in the format of opening tag - content - closing tag.
Tags can also have attributes, which are extra bits of information. Attributes appear inside the opening tag and their value is always inside quotation marks. They look something like <tag attribute="value">Margarine</tag>. We will come across tags with attributes later.
Tags tend not to do much more than mark the beginning and end of an element. Elements are the bits that make up web pages. You would say, for example, that everything that is in-between and includes the <body> and </body> tags is the body element. As another example, whereas '<title>' and '</title>' are tags, '<title>Rumple Stiltskin</title>' is a title element.