There are two principal types of U.S. patent applications that can be filed; provisional applications and regular applications. As soon as you file either type of application, you are "patent pending".
Most patent applications are nonprovisional or "regular" applications, meaning that they include a detailed description of the invention, drawings, and claims. Claims are actually the most important part of your patent because they define what you own. Think of claims in the same way as you think of the property line where you live. Everything inside the claims is yours; everything outside of the claims belongs to someone else. Also, everything in the claims must be covered in the detailed description of the invention.
A regular application is examined by the United States Patent and Trademark Office, and can mature into a patent. If you would like to file foreign patent applications based on your U.S. application, you should file your foreign applications within one-year after filing your U.S. application.
You may have heard about a type of application called a "provisional" application. A provisional application is never examined by the United States Patent and Trademark Office, and automatically expires one-year after it is filed. Also, claims are not required in a provisional application but there may be circumstances where claims should be included. In order to obtain a patent, a nonprovisional (i.e., "regular") application must be filed within one-year after the provisional application is filed.
In some circumstances, a provisional patent application may cost less to file than a regular application because it can be filed in a less polished format than a regular application. Therefore, it can be a faster and possibly lower cost way to become patent pending. However, the overall cost of the patent process will be typically be higher than if you filed a regular application to begin with.
Provisional applications can be riskier to file than a regular application.
For example, since a provisional application does not have to be filed with claims, how do you know if your description of the invention will be sufficient to cover the claims you write a year later when you convert your provisional application into a regular application? You won't unless your provisional application is as detailed as a regular application, and perhaps more detailed. Also, there is the risk that a provisional application will not give you the priority that you are seeking if the application is not sufficiently complete.
Why file a provisional application?
If you are about to publish a detailed technical paper that describes your invention, and there is not enough time to file a nonprovisional application, you may have no choice but to file a provisional application to prevent a loss of foreign patent rights. In that case, it may be possible for your patent attorney to file a the technical paper as the provisional application.
Or, if you are about to sell or advertise a product, or make a public disclosure of your invention, and there is not enough time to file a nonprovisional application, you may want to have your patent attorney file a provisional application as an attempt not to lose your patent rights.
In other words, provisional applications should be filed with a particular strategy in mind, and not simply as a way to file a lower cost patent application. The cost savings could result in a loss of patent rights.