HTML is the language web pages are written in. There are other languages used with HTML also such as Javascript, PHP, AJAX, Java and others.
HTML was created with the creation of the world wide web (where the www that is on the front of many web/Internet addresses comes from) and has become much more robust over the years. Currently version 4 is being used but 5 is starting to be used and will see heavy use in the future.
Basically in HTML you use tags to mark up (or fancy up) the text and pictures. HTML is an acronym for Hypertext Markup Language. It is not a language like many other computer languages such as Fortran, COBOL or BASIC that are steps telling the computer what to do. In HTML the tags in the code tell the computer how to make something appear usually.
Tags in HTML appear like <b> or <ht> or <p> or <img src="mypicture.jpg"> If you were to look at the code for a page (you can look at code for pages by either choosing View Source in the Tools menu or right clicking on the page and choosing View Source) it may look like Greek to you but if you will stop and look you will notice the text you were looking at is there and you will see lots of tags: code in <>. <b> tells the computer to bold any text that follows. The way to stop something you started (like the bolding) is to use the same tag but it has a / in front of the code so to stop the bold you would use </b>
To make the computer put the next text on a new line and skip a blank line you use the </p> tag with the p standing for paragraph, indicating you finished the paragraph. So if we wanted to bold my name and then put under it computer author we would use this code: <b>Dwight Watt</b><p>computer author</p>
That is a real quick and trying to not to get to geeky explanation. If you want to get some more basics of HTML on my articles page on my web site is three chapters of introductory HTML I wrote.