HTML5 tutorial - HTML is the predominant language of the World Wide Web.

Since its inception HTML has been the predominant language of the World Wide Web. Indeed, no small part what made the World Wide Web "World Wide" was HTML. With the creation of HTML and the web browsers that interpreted it anyone with a computer and a phone line could surf the web *.

There are other languages (e.g. XML) used to write web pages, but most web sites are written in HTML and will be for the foreseeable future.

Essentially:
HTML is how two computers speak to each other over the internet.
Web sites are what they say.

HTML is "spoken" by two computers:

1. Client
The client is used by the person surfing the net, such as the computer you are looking at right now.
2. Server
A server stores and distributes websites over the net. I have a server where I store this web site.

The terms are often encountered in the phrases "server-side" and "client-side"; meaning something occurs on the server or the client. "That script runs client-side." means "That script runs on the client".

Let's assume that someone wants to buy a doohickie, and as luck would have it you sell doohickies.

This potential buyer turns on his computer – the "client", opens a browser, goes to Google, types in "doohickie", clicks "Search" and (in a perfect world) a link to your site comes up at the top of the search results.

This person, with credit card in hand, clicks the link to your home page. The "client" (the computer, not the person) sends a request to the "server" to send your home page back to the "client".

The server finds a file named "index.html" or "index.htm" ** (the default name of the home page) and sends it to the client. The index file in turn requests whatever additional files (e.g. pictures) needed to display your home page in all its glory.

Index.html is written in HTML, the latest version of which is HTML5.

HTML

Hypertext Markup Language - history
wikipedia

web browser

e.g. Mosaic, Netscape, Internet Explorer, Firefox, Chrome, Safari etc.
google · wikipedia

modem

A voiceband modem turns the digital data of a personal computer into analog audio signals that can be transmitted over a telephone line.
wikipedia

XML

Extensible Markup Language
wikipedia · w3.org

HTML5

Hypertext Markup Language ver. 5
google · wikipedia · w3.org

URL or Uniform Resource Locator

A Uniform Resource Locator (URL) specifies where an identified resource is available and the mechanism for retrieving it. For example, the URL of this page is:
http://www.html-5-tutorial.com/about-html.htm
wikipedia · about.com

* For all of you too young to remember, those intrepid souls who first surfed the net oh so long ago did so via a dail-up connection that ran through a modem hooked up to a phone line.

** There is no difference between a file with an ".html" or ".htm" extension. I use the latter because there is one less letter to type.
To confuse matters even more, many sites (including this one) hide "index.html" or "index.htm" in the URL of the home page. For example: http://www.yahoo.com/index.html goes to http://www.yahoo.com/. The "index" file loads just as if it were there, you just don't see it in the URL. It helps a bit with SEO, but mostly it just makes for a cleaner URL.