Hyperfocal distance is distance to which when focused (at a particular focal length and f-stop) the scene will be focused from 1/2 that distance to infinity. At least that is the theory. It all depends on one's hunger for absolute sharpness and on the size of the print. The larger you print, the more depth of field you will need (if overall sharpness is what you are after).
Old fixed focal length lenses had depth of field scales right on them, and those could also be used to focus the lens to the hyper focal distance based on selected aperture. These days,especially with zoom lenses, that is not so. You can either remember it for the few most commonly used focal lengths, create or download charts or wheels, or use apps for your smart phone. One example of an app that helps with this task is Simple DoF. For my 18-200mm lend on a D200, I got the following matrix:
|Aperture||Hyperfocal Distance||Range of Focus|
|5.6||9.5 feet||5 - infinity|
|8||6.6 feet||4 - infinity|
|11||4.8 feet||3.2 - infinity|
|16||3.3 feet||2.5 - infinity|
|22||2.4 feet||1.9 - infinity|
DOF Master is a great reference page that will further explain all of these topics and also allow you to create some useful tools for yourself.
To see for myself how this changes the scene in a real life situation, I set my camera on a tripod in a local park, placed some broken branches and pine cones in the foreground, and allowed the scene to run way into the distance ending with a line of trees. Below, you see photos shot from f/5.6 through f/22 with the lens at 18mm. No sharpening done in SW.
18mm - f/5.6
18mm - f/8
18mm - f/11
18mm - f/16
18mm - f/22
Below, you will find a filmstrip of a 100% crop from the background from between the two trees to better see how well the background truly was sharp.
As I said at the beginning, the hyper focal distance is driven by the final print size. Looking at the 100% crops would require rather large prints to make the areas as big in the print. So from that point of view, the results can be deceiving. However, it also shows that it's all truly relative. With a scene that has no major elements in the very foreground or background, like a field of grains, hyper focal distance can be a great fit. Is there a mountain range in the background or a city skyline? Hmm, I am not sure about the use of hyper focal distance in those instances.
In those situations it might actually be more beneficial to resort to the alternate hyper focal distance definition, which has you focus the lens at infinity and tells you the distance beyond which all objects are acceptably sharp. But obviously, the background would be the sharpest. There's about a stop of a difference between the two definitions.
At 18mm, the depth of field is rather great even at f/5.6. By focusing to about 10 feet, everything from 5 feet to infinity is in acceptable focus. That's the optimistic view. As you can see from the filmstrip, it's questionable what acceptable means. However, from the photos themselves, , it is rather clear that only a very large print would reveal this lack of sharpness in the distance. FUrther, please, consider that my line of background trees was not exactly i infinity but rather significantly closer. The same experiment with an object on the horizon would result in an even lower infinity sharpness.
As such, always consider the circumstances. What area needs the most critical focus? Is there an area that will look just as good if not in full focus, like distant peaks in fog? If still in doubt, bracket your focus. Try straight blind hyper focal distance, infinity focus, and focus on a specific element.
Beyond what I touched on here, things get even more complicated once you start considering diffraction, which causes overall softness of photos taken at very small apertures and thus works against you when trying to use f/22 for maximum depth of field. But that is for another day.