WHAT A PATENT IS (AND WHAT IT IS NOT)
A patent is a grant by the federal government to the patent owner (the "patentee") of the right to exclude others from making, using, selling or importing the invention.
When most people think of patents, they think of utility patents, which are patents issued for the functional aspects of products and processes (there are two other kinds of patents, design patents and plant patents, which aren't discussed here).
Patents do not protect mere "ideas". Rather, they protect structures and methods that apply technological concepts.
A patent is not a grant of a right to use the patented technology. Rather, it is a right to exclude or prevent others ("infringers") from using the technology defined in the patent claims. The patentee might not be able itself to use the technology of the patent because it is merely an improvement on technology previously patented by another. In that event, a license from the previous patentee is needed.
A U.S. patent is enforceable against infringers from the date it is issued by the Patent Office until it expires (generally twenty years from the date a patent application is filed).
A U.S. patent is presumed to be valid, and that presumption can be overcome in litigation only by clear and convincing evidence presented by a challenger of the patent.
A useful invention may be patented 1) if it is "novel", meaning that it was not anticipated by being identical to technology disclosed in a single piece of "prior art" as defined in the Patent Code; and 2) if it is "non-obvious", meaning that the technology, although new, is also different enough that it is not obvious in view of the prior art.
"Prior art" under the Patent Code includes inventions made by others prior to the date the invention in question was made. It also includes, however, acts by the inventor himself, such as offering the invention for sale or displaying the invention publicly, if such acts take place more than a year before the inventor files a patent application.