HTML or Hyper-Text Markup Language is the language, or coding to make things display, that is used on the web. It is also used to make simple apps and programs in some cases, as well as being used to create help files for software among many other uses.
HTML5 is the standard covered by this tutorial as it was the current version at the time of writing.
HTML is used to produce a result on a client's display. It is written in a way that the browser on the client's computer can understand and interpret, which it then transforms into results. In effect, all the data that is produced by the HTML is painted on the client's display.
What HTML is and what it is not
HTML stands for Hyper Text Markup Language - it is a markup language.
When writing a Java program, it is called programming. When writing HTML, there is a general misconception that this is programming. It is an important point to remember however, HTML is not a programming language. All that HTML is designed to do and does is provide the browser on the client side with rendering instructions and the browser will then use that information to render the page.
How HTML is useful in web development
HTML is the fundamental markup language of the web. There are of course other markups, particularly the more attractive and strict XML and it's friends and family such as XHTML. Both of these are attempts to abolish HTML as a standard and replace it with an XML serialisation of the web. XHTML is understandable as it is more strict than HTML (unless following strict application), but XML is another story.