The description in a patent application must enable other people to make and use the invention. One cannot keep some aspects of the invention secret while attempting to patent it; full disclosure is required.

A good description identifies the problem addressed by the invention, acknowledges which elements of the invention are already known, and clearly describes the new element(s).

New matter cannot be added to an application after it has been filed.

Patent drawings must meet strict requirements for line weight, lettering size, page size and the like. The drawings must be consistent with each other and with the description. For inventions which can be illustrated, every element recited in a claim must be shown in the drawings.
Claims are the legal definition of an invention, and therefore the most important part of a patent.

Claim drafting is an art requiring obedience to arcane principles including single-sentence construction, antecedent basis, explicit recitation, and claim differentiation. A good claim clearly identifies what is new in the invention that distinguishes it from the prior art.

Claims may be amended after filing, but they cannot go beyond what was shown in the original drawings or described in the original description.

The Abstract is a short summary of the invention. Its purpose is to give searchers and examiners a quick understanding of the invention, once the application is published.
An Application Data Sheet is a special form prescribed by the PTO in which inventor data and other pertinent information is reported by the person who files the application.
The I.D.S. is an important part of most applications. It reports documents and information known to the applicant or his attorneys which may be pertinent to the question of patentability. Failure to reveal such information to the examiner may result in an inability to enforce the patent against infringers. See candor.
A declaration signed by each inventor is required in every case, with the exception of cases where an inventor is unavailable or refuses to sign, in which case the other inventors, or the owner of rights to the invention may sign of that inventor's behalf.
Recording an assignment is optional. If no assignment is filed, the patent will be granted to the named applicant which, for applications filed before September 16, 2012, is the inventor(s). If an assignment is made before the issue fee is paid, normally the assignee's name will appear on the face of the patent. Post-grant assignments can also be made.