A patent is an exclusive right to exploit an invention, i.e. a technical solution to a problem. A patent can be valid for a maximum of 20 years, but for some medicines and pesticides it is possible to get another five years of protection.
The exclusive rights obtains by registering a patent imply that no one may use the invention for professional purposes, for example; using, manufacturing, selling or importing the invention without your permission. When the patent expires, so does your exclusive right. From then on, anyone is free to use your invention.
The purpose of the patent system is to encourage technological development in society. As an inventor, you are rewarded with a patent in return for permitting the State to make your invention public. That way, other people have the opportunity of learning from it and developing it further.
Not only pioneering inventions, where all the elements are new, can be protected. The vast majority of patents concern further developments of known technology.