What is a Provisional Patent Application?
Practitioners will typically shorten this to simply a "provisional application" or just a "provisional". The idea behind a provisional application is to hold the inventor's "place" in the first-to-file line.
Prior to March 16, 2013, the US was under "first-to-invent" rules. However, the provisional application still held the inventor's "place" to prevent prior art from rendering the invention unpatentable.
Twelve months after filing the provisional application, an inventor may file a "non-provisional" application, which is a roundabout way of saying: a true application that's actually examined and leads to a US Patent that can be infringed.
There is no such thing as a provisional application for a design patent.
What Happens if a Non-Provisional is not Filed in Twelve Months?
Because the provisional is held in confidence, not examined or published, it won't be a prior art reference to any later application. This means that if you file a Non-Provisional outside the twelve month period, your provisional filing can't be used against you. It also means that your provisional filing can't be used against your competitors.
Additionally, and more importantly, the inventor loses her place in line. If someone else has a priority date before the later Non-Provisional, they may be entitled to the patent under the first-to-file rules. Additionally, if a piece of bombshell prior art comes out later, it could wipe out any patent on the subject matter.
Can Someone be Sued Based on a Provisional Patent Application?
No, but with qualifications. If you file a provisional application and do nothing else, there's no legal basis to sue someone for infringement because you usually can't infringe a non-published application, and provisional applications are never published.
However, putting your competitors on notice of your intent to get a patent can be a useful planning tool. Making it easy to keep tabs on your patent applications means that there's a higher likelihood that, in a hypothetical infringement trial you could make a case for willfulness damages, which can triple the damage award.
On a later-filed, Non-Provisional application, a lawsuit can start after the patent has been issued but damages may, under certain circumstances, be calculated from when the application was published.
How does the Provisional Relate to Applications Filed Afterwards?
The twelve-month period is still the most important thing to keep in mind. Any subject matter that you want to hold a place in line for must have a corresponding Non-Provisional filed within twelve months or the place is lost.
Thus, it's possible to do things like:
file a provisional directed to A,
2 months later file another provisional directed to A+B,
2 months later file a provisional directed to A+B+C, and
8 months later (today) file a non-provisional directed to A+B+C.
This results in the following priority dates:
A: 12 months ago
A+B: 10 months ago
A+B+C: 8 months ago
So the basic principle is that the inventor can only hold her place in line with respect to the subjects disclosed in the provisional. However, she can also file subsequent provisionals on improvements to the first provisional.
Remember that the USPTO has strict rules as to how priority is claimed. While it is legal and not unheard-of for inventors to do this themselves, I would not recommend to anyone, including a registered practitioner, that they prosecute their own case without an attorney or agent.
If I Need an Attorney/Agent, But I'm Short On Cash...
When I was in law school, I worked for the JMLS Patent Clinic, which is one of many patent clinic programs authorized by the USPTO to provide services to inventors who may not be able to afford it. All work at the JMLS clinic is supervised by a registered patent attorney.