As defined by the United States Patent and Trademark Office (USPTO):

A US patent for an invention is the grant of a property right to the inventor, issued by the Patent and Trademark Office. A patent refers to a right granted to anyone who invents or discovers any new and useful process, machine, article of manufacture, or composition of matter, or any new and useful improvement thereof. Moreover, a patent is a property right that gives the patent holder the right, for a limited time, to exclude others from making, using, offering to sell, selling, or importing into the United States the claimed invention in exchange for a full disclosure of the invention. In simplified terms, a patent is a way of protecting an invention (any new and useful process, machine, manufacture, or composition of matter, or any new and useful improvement thereof).

Therefore, without a patent, anyone can legally make, use, sell or import your invention.  Note that ideas are not patentable, but rather the knowledge of how to make and/or use the idea is patentable. To be patentable the law specifies three primary patentability requirements wherein the subject matter of the invention must be useful, novel and non-obvious.

Three hurdles to Patentability

Useful means to have utility as in an invention must be capable of some beneficial use.  Novelty means the invention must be new and an invention, in general, is not new if it has been described in a printed publication, known or used by others, or has been in public use or on sale (what is known). If an invention is not new, then the invention is not patentable. Non-obviousness means if the differences between the invention sought to be patented and the prior art (what is known) are such that the invention as a whole would have been obvious to a person skilled in the subject of the invention then the invention is obvious and not patentable.  As an example obvious non-patentable subject matter includes the following obvious changes substitution of one color for another, or changes in size, or insignificant changes or improvements over what is known.

Patents come in three types – Utility Patents (cover new and useful inventions such as mechanical devices, electronics, medical devices, biotechnology, gadgets, and processes for making things). Design Patents (cover new and ornamental designs of products (articles of manufacture) such as containers, furniture, toys, or house wares). Plant Patents (cover new and distinct plant varieties such as flowering plants, vegetables and fruit trees).