What is HTML?

When you write a normal document using a word processor like Microsoft Word/Office, your text is saved in a file with a special format. It is not simply saved as the string of words you typed since the document needs to preserve things like the font you chose, the size of the text, which words are in bold, which italics, and so on. The special format includes not only your words, but all these extra information so that the next time Word opens your document, it can display the document with the exact appearance you created earlier.
In the same way, web pages are simply strings of words put in a special format that web browsers are able to display. While the format of Word documents is simply called "Word format" (or "doc format"), loosely speaking, one might say that web pages are formatted using "HTML". Take the paragraph of text in the box below for example:

This is an example paragraph to illustrate what HTML is, for the purpose of explaining common terms like HTML, JavaScript and PHP.

If you were to peek into the raw code for the above words, you will see the following:

This is an example paragraph to illustrate what HTML is, for the purpose of <a href="http://www.thesitewizard.com/html-tutorial/what-is-html.shtml">explaining common terms like HTML, JavaScript and PHP</a>.

Notice that it is more or less like the text given earlier, except that there is additional information embedded. For example, the portion that says <a href="http://www.thesitewizard.com/html-tutorial/what-is-html.shtml"> (which I placed in a different font above to make it easier to spot) tells the web browser that what follows, until </a> is reached, is to be regarded as a link pointing at the web address http://www.thesitewizard.com/html-tutorial/what-is-html.shtml
When the web browser sees this information, it makes the words "explaining common terms like... [etc]" appear as the blue underlined text that represents a clickable link. The rest of the text is just displayed as-is.