This has two section: drawings and the specification
 
Because the drawings are part of the description, claims may be based on what is shown in the drawings, even if not described in the specification. Drawings are not always included as a part of the application. They are only used if necessary to understand the invention.
 
In patents claiming a mechanical apparatus, drawings are almost always present. The drawings show the various parts of the invention, each of which is assigned a reference number that is defined in the specification.
In biotechnology or pharmaceutical patents, drawings are not always included. In a patent that claims a combination of ingredients used to make a pharmaceutical preparation and a method of making and using the combination of ingredients, a drawing would not be necessary to understand the patent.
 
Drawings encountered in biotechnological and pharmaceutical patents include gene or amino acid sequences, schematics of plasmids or recombination schemes, electrophoresis gels and chemical formulas. These types of figures might not need identifying numerals
 
The specification is the part of the patent that describes what the invention is, how to make it and how to use it.
 
It also discusses the prior art and how the invention overcomes the problems that were unsolved before the invention. Prior art documents, such as patents or scientific articles, are discussed to show how previous efforts have failed to provide an answer to problems that are addressed in this patent.
 
A summary of the invention is also included in the application. This discusses the invention in broad terms. The figures/drawings are also described in broad terms, typically in one sentence e.g. “ Figure1 shows a side view of the apparatus of the invention” or “Figure 2 shows the nucleotide sequence of gene X”.
 
This is then followed by a detailed description of the invention, such that someone “skilled in the art” would know how it could be done. This is the section that takes the most careful writing as it is often felt that the invention should be described without the final “ inventive step” being disclosed.