Patentable inventions are new and useful machines, processes, manufactures, compositions of matter, and improvements thereof.
A machine is a device having mechanical and/or electrical components invented to perform a useful function. We use machines everyday. Examples of machines are: cars, tools, computers, and games.
A process is a series of steps that, when followed, manupulate material or information and produce a desired result or product. A process may be the steps involved in the use of a new device or the steps involved in a new use of an already existing device.
Inventions that are compositions of matter are typically compounds or alloys. Table salt, Sodium Choloride, is a compound and would be considered a composition of matter. Similarly, steel, which is an alloy of iron with impurities such as carbon, is an alloy and also would be considered a composition of matter.
A manufacture is a new and useful item that is not a machine, process or composition of matter.
The patent laws also provide for patents covering improvements of already existing technology. This means that an invention does not have to be entirely new.