The patent pending phase starts when your ordinary patent application or provisional patent application is filed and lasts until the patent issues. Throughout the patent pending period, your rights depend upon whether you have filed a Non Publication Request (NPR). If you have not filed an NPR, the PTO will publish your ordinary patent application 18 months following the filing date. When it is published, you acquire provisional rights that permit you to obtain royalties from an infringer for actions that occurred from the date the infringer gets genuine notice of the published application. (You can provide actual notice to the infringer by conveying a copy of the published application by certified mail, return receipt requested.) You must wait until after the patent issues to request these "patent pending" royalties. If the patent does not issue, you cannot acquire any royalties.
If, at the time of filing your application, you filed an NPR, your application will not be published prior to issuance and you will have no rights during the patent pending period. In other words, if it is not published prior to issuance, anyone can liberally make, use, sell, and present your invention for sale during the entire pendency period.
In general, a probable infringer won't duplicate an apparatus that is known to be patent pending. This is because the infringer would seek the possibility that a patent will later be issued and you'll use your patent to enforce your protective rights. This means, to stop any further production and marketing. In this case, the funds the infringer would have to spend on the costly tooling will have been wasted. Another reason for showing a device "patent pending" is to show that you have given notice to probable infringers, thereby giving you the right to obtain damages and attorney fees (after the issue of your patent) for the determined infringement.
After your application is filed, you may issue articles on your creation without damage of any legal rights in the U.S. or foreign Convention countries but you'll lose rights countries that are not under the convention. However, it's not advisable to disclose details of your invention to potential competitors at this early stage, especially since your application may not have become a patent.