If you’re writing a particularly long article (which hopefully is long because it’s complex, not because you’re babbling on, or trying to achieve a preset number of words), you might want to split it into several pages to make it less overwhelming for your readers.
Naturally, being the awesome CMS that WordPress is, it carries this functionality natively. And there really isn’t a whole lot to it. Just find the point where you want to add a page break (there can be several), and type in the following:
The default appearance of the page links at the bottom of the page is very basic, but does the job. However, if you want a bit more control over how it looks, you can wrap your page links in a div tag. There’s more on this in the Codex if you’re interested.