Most of us are familiar with word-processing programs. They're mainly designed to help prepare pages of text for printing. Word processors offer a lot of fancy formatting features that allow the user to fine-tune every aspect of the page: margins, fonts, size of text, and so on.
Text editors are another breed of animal. Text editors allow the user to create and manipulate the content of a text document, but without the fancy formatting features mentioned above.
You probably have a very simple text editor on your hard drive right now, even if you don't know it.
Most Windows machines come with a program called Notepad. Fire it up and you'll discover that it's very simple indeed.
Mac users can search their hard drive for a program called SimpleText. (In my experience, they are likely to find several copies.) This program actually does offer a few basic formatting features, such as font, style and size.
Either of these programs will allow you to generate simple, text-only files. I recommend using them to get started learning about HTML. Save your files with the .html (or .htm) extention. More extensive instructions are given in the tutorial.
But, unless you enjoy typing a lot of code by hand, neither of these programs is very efficient for marking up documents for the Web. You will probably want to use a more advanced tool for serious Web authoring.