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”.