A provisional patent application is a patent application that is not examined by the patent office, but has the effect of reserving your filing date (also known as the priority date). A provisional application needs to be followed up by a non-provisional filing (within one year), which is examined by the patent office. A provisional application has no formal filing requirements, but still needs to be in as much detail as possible.
While there are no formal requirements to file a provisional patent application,it is important that a patent attorney strategizes and plans before filing drafting the application. The advantage of a correctly filed provisional application is that the non-provisional application (when filed) can piggyback to the filing date of the previously filed provisional application. Be aware: a mediocre provisional patent application filing can do more harm than good and can even cause invalidation of a patent if one is granted (see New Railhead Mfg., L.L.C. v. Vermeer Mfg. Co., 298 F.3d 1290, 1294 Fed.Cir.2002).