provisional patent application (PPA). The PPA preserves your place in line at the USPTO and it may serve as a deterrent but it's not a patent. You cannot use a PPA to stop others from copying your invention, and it only has value if you file a regular patent application within one year of filing the PPA. Because less than half of patent pendings "matriculate," into patents, a lot of companies are wary of patent pending status.
(BTW, there's a lot of helpful patent business info at Nolo's new patents and business site.)