This eponymous section of a patent actually needs more explanation than one might garner from its name alone: the detailed description goes beyond a description of the invention. The description is the primary means for providing support to the claims, offering explanation, giving body to the patent. The description lays out how the invention is made and how it is used in sufficient detail to enable someone in the invention’s field of industry to enable them to also make and use it. It teaches the public how to build and use this thing you’ve invented and is essential in the quid pro quo that the patent system represents.
The description generally is also the part of the patent that describes the best mode – sort of the best form of the invention – although it need not point out the best mode. There is no requirement that the patent specifically flag the best mode; it just has to be in there.
The description also provides the patent applicant to become “his or her own lexicographer;” to give words special meanings that may be uncommon or different from their general understanding.
Often, the detailed description follows the drawings from figure to figure, explaining what each shows with great specificity. The description will sometimes set out multiple embodiments of the invention, even if those embodiment aren’t claimed later on. And most patent practitioners show their apathy protect the breadths of the invention by including phrases like “many variations on the invention will be obvious to one having skill in the art and are within the scope of this disclosure but will not be repeated here.”