The language HTML is derived from. It places most of the control over the appearance of the document in the hands of the program viewing it. The idea behind this is to allow presentation of the same data in a variety of different media (print, computer, braille) in a fashion most appropriate to that medium.

So instead of specifying the font and size you want a header (such as the one above) to be, you would simply place a <TAG> around the text indicating it is to be displayed as a header.

Here is a more detailed explanation.