Well in addition to the above here are some excerpts from USPTO website (of course exact rules may vary from country to country):
1. An applicant who files a provisional application must file a corresponding non-provisional application for patent (non-provisional application) during the 12-month pendency period of the provisional application in order to benefit from the earlier filing of the provisional application. However, a non-provisional application that was filed more than 12 months after the filing date of the provisional application, but within 14 months after the filing date of the provisional application, may have the benefit of the provisional application restored by filing a grantable petition (including a statement that the delay in filing the non-provisional application was unintentional and the required petition fee) to restore the benefit under 37 CFR 1.78. So, the benefits of the provisional application cannot be claimed if the 12 month deadline for filing a non-provisional application has expired (unless the benefit of the provisional application has been restored under 37 CFR 1.78).
2. The provisional application must name all of the inventor(s). In view of the one-year grace period provided by 35 U.S.C. 102(b)(1) in conjunction with 35 U.S.C. 102(a)(1), a provisional application can be filed up to 12 months following an inventor's public disclosure of the invention. (Such a pre-filing disclosure, although protected in the United States, may preclude patenting in foreign countries.) A public disclosure (e.g., publication, public use, offer for sale) more than one year before the provisional application filing date would preclude patenting in the United States.
3. Beware that an applicant who publicly discloses his or her invention (e.g., publishes, uses, sells, or otherwise makes available to the public) during the 12 month provisional application pendency period may lose more than the benefit of the provisional application filing date if the 12 month provisional application pendency period expires before a corresponding non-provisional application is filed. Such an applicant may also lose the right to ever patent the invention.