By statute, you can only patent something that falls into one of these four categories: a process, a machine, an article of manufacture, or a composition of matter.
In plain language – you can patent a way of doing something, a machines that does or makes something, an object that is manufactured, or molecules such as drugs and proteins.
There are, of course, additional restrictions. If your process is nothing more than a series of mental steps, it cannot be patented. If your invention boils down to nothing more than a scientific or mathematic principle, it is not eligible for patent protection.