Those readers who have been around computers for a few years may remember when wordprocessing documents needed to have special markers inserted to turn boldface printing on and off. These markers didn't print, but controlled how the document was displayed by the printer. These markers are invisible when you see the page through the eyes of the browser software. Most tags contain just a letter or two, although some may be much longer. While the browser doesn't care if the letters in the tag are upper or lower case, upper case letters are easier to recognize when looking over your document.
Every Web page has the same basic structure.
That's all there is to a Web page, and it's easy to understand if we take it apart bit by bit. This tag just lets the browser know that it's dealing with a Web page. It's sort of a warning to the browser that some HTML follows, and is the first tag in all Web pages. The HEAD tag identifies the first section of the Web page document. The title is the text which appears in the title bar at the top of the window.