So just what may be patented?  
The requirements for patentability are outlined in 35 U.S.C. 101.

This statute states that "anything under the sun that is made by man may be patentable". After reading that, you might think that includes anything and everything.

From the statement above it may seem that way, but for starters, this isn't the only law. An invention must be novel (covered in 35 U.S.C. 102), nonobvious (covered in 35 U.S.C. 103) and its specification must be up to par (as required by 35 U.S.C. 112). In addition, under 35 U.S.C. 101, an invention must have practical applications and produce a "useful, concrete and tangible result".

Upon taking a closer look, there are many objects that are excluded from being patentable due to the phrase "made by man" taken from 35 U.S.C. 101. For instance, naturally occurring objects are never patentable. That means if you find a new mineral deep within the earth, you will not be able to patent it. The same goes for plants found growing naturally in the wild.

Abstract ideas like the laws of gravity are not patentable either. The subject matter must be more than an idea or a concept to be considered for a patent. Hopefully you're catching on to the idea that patentable objects must be a product of human ingenuity. Furthermore, patentable objects must have a distinctive name, characteristic, and use.

With regards to living organisms, the PTO has decided that living materials modified by human intervention may be patentable. However, human organisms, including human embryos and fetuses are not patentable.

More on patentability  
There's another important point or two concerning patentability. Descriptive materials are never patentable unless they have a function. For example, a computer program that instructs a computer to perform a particular action may be patented.

On the contrary, instructions or materials lacking a function may not be patented. This means that music, literary works and lists of data are not patentable. These are all termed nonfunctional descriptive materials.   Even when nonfunctional descriptive material is recorded on a computer-readable medium it is not patentable.

Categories of inventions
There are four main categories of inventions. These are machines, manufactures, compositions of matter and processes.

Machines are defined as concrete things consisting of parts or certain devices that may be combined. Furnaces, copiers, cell phones and can openers are all examples of machines. They are considered fully reduced to practice when it is assembled, adjusted, and used.

Manufactures are created from raw or prepared materials that through some sort of production process (like hand labor or machinery) are given new forms, qualities, properties or combinations. A paper clip is a manufacture, as is a wine bottle. An invention in this category must be completely manufactured to be considered in a state ready for patenting.

Compositions of matter are the result of a chemical union or a mechanical mixture between two or more substances. Paint is one such example. Even better, a therapeutic drug, whether liquid or pill form is also considered a composition of matter. A composition must be completely composed before it can be considered ready for patenting.

Finally, we come to the processes. These involve actions, like a method or a series of steps. Purifying and sequencing DNA are processes that may be patented. Processes are considered fully reduced to practice when successfully performed.