A patent can de divided into the following sections:

1. Abstract. This is a one-paragraph description of the invention that's being patented.
2. Drawings. These are carefully labeled figures that are used to illustrate important concepts of the invention and that are described in the detailed description. It is important that at least one diagram shows a block diagram of the invention if the patent includes apparatus claims. For a physical device, a diagram should show the physical parts and their relationships. For software, the diagram should show the architecture of the software. For a patent that includes method claims, at least one diagram should have a flow chart that illustrates the method.
3. Background of the invention. This section describes the field of the invention, other inventions, or publications that predate this invention ("prior art"), and other inventions related to this invention. This section typically explains the inventions usefulness-what's so great about the invention and why people will want it.
4. Summary of the invention. This is typically one paragraph to one page in length that describes the invention. Though this section isn't legally required by law, it's a part of almost every patent.
5. Brief description of the drawings. For each drawing in the patent, there should be one or two sentences to briefly describe it.
6. Detailed description. This section explains the invention as completely as possible, referencing the drawings. This section describes what is called the "embodiment" of the invention that differentiates it from existing inventions. The description must explain the best way the inventor knows of implementing the invention, and it must be detailed enough to allow "one of ordinary skill in the art" to produce it. Remember that the patent must be novel, nonobvious, and useful.
7. Claims. This is the essence of the invention. Each claim is a single sentence, though it's usually a very long sentence broken into multiple parts, that describes the invention in as precise wording as possible.

There is often debate among patent attorneys, inventors, and patent litigators about the definition of "one of ordinary skill in the art." The Manual of Patent Examining Procedure (MPEP) section 2141.03 defines one this way:

The person of ordinary skill in the art is a hypothetical person who is presumed to have known the relevant art at the time of the invention. Factors that may be considered in determining the level of ordinary skill in the art may include:

(1) type of problems encountered in the art;

(2) prior art solutions to those problems;

(3) rapidity with which innovations are made;

(4) sophistication of the technology; and

(5) educational level of active workers in the field. In a given case, every factor may not be present, and one or more factors may predominate.

Note that the definition of one of ordinary skill in the art will vary depending on many factors and cannot be defined generally, but must be defined for each particular patent.

Claims are the most important part of any patent. Patents typically contain anywhere from 5 to 30 claims. Each claim is a single sentence, and each describes an important aspect of the invention. Whether another product infringes on a patent ultimately comes down to these claims.

One last piece of advice: never file a patent that you've written yourself without having a qualified patent attorney or patent agent review the claims. I've seen patents for ingenious devices that were worthless because of one misplaced word in a patent claim. Other patents had no value because the claims were so narrow that no one infringed, or avoiding infringement required some insignificant change to the invention.