In 2009 alone, the U.S. Patent and Trademark Office received nearly half a million patent applications and granted about 40 percent of them [source: USPTO, "Statistics"]. Each of these applications requires detailed text descriptions and drawings of the invention. When you do your patent search, this is the first documentation you'll find.
Each patent may also reference other patents and non-patent literature that describe the prior art closest to the invention. Prior art is technology related to the new invention that people had previously described in a publication (such as a science journal) or made, sold or used for the same purpose as the invention. For example, suppose you're inventing a new coffee grinder. Prior art would include existing coffee grinders that operate in a similar way but don't include your new improvements. Prior art could also be some newly discovered grinding process your invention is using that was recently published in an engineering magazine. Depending on what you're looking for, you might want to examine the referenced documents describing this prior art.
No matter what kind of patent search you undertake, knowing how all these documents are categorized can help you narrow your search. The agency uses its U.S. Patent Classification System (USPCS) to stay organized. The USPCS groups patents into classes that identify some major category of objects, including everything from apparel (class 002) to telecommunications (class 455). Each class includes subclasses, and each subclass can include other subclasses. This hierarchy might be several levels deep before it is precise enough to classify a particular patent, and the USPTO creates new subclasses as needed for even more precision [source: USPTO, "USPC"].
When reading the classification on the patent itself, you'll notice a class/subclass pair using a slash. For example, "2/456" means class two and subclass 456. For a complete list of all the current classifications in the USPCS, see the USPCS index at USPTO.gov or the Manual of Classification (MOC) published by the USPTO.
If your search takes you outside the U.S., you should be aware that international patent agencies use different classification systems. For example, countries in Europe use the International Patent Classification (IPC) and the European Classification (ECLA). Some examples of international patent organizations include the Japan Patent Office (JPO), the Korea Intellectual Property Rights Information Service (KIPRIS) and the State Intellectual Property Office (SIPO) of the People's Republic of China. U.S. patents that are also filed in other countries list the patent's international classifications, which saves you time in your search.