A patent application will be examined to ensure that it complies with the patent laws and rules before it can issue as a patent. The most commonly cited laws in rejections from the Patent Office are 35 U.S.C. 101, 102, and 103.
35 U.S.C. 101 - Under section 101, the claimed invention must be "a process, machine, manufacture, or composition of matter, or any new and useful improvement thereof." This rule is rarely ever cited by the Patent Office as a basis for a rejection outside of the software and business method classes of patents. As of this writing, software and business methods are still patentable provided they are tied to a particular machine or are transformative.
35 U.S.C. 102 - Under section 102, the claimed invention must be novel. An invention can be rejected under section 102 if the Patent Office can show that there exists a single prior art reference that discloses the invention either expressly or inherently exactly as it is recited in the claims. For example, assuming that the claims of an application recite a device that includes the elements A, B, and C. a rejection under 102 is proper in this instance if the Office can show all of the elements A, B, and C are disclosed in a single prior art reference.
35 U.S.C. 103 - Under section 103, the claimed invention must not be obvious. An invention is obvious under section 103 if the Patent Office can show that at least two prior art reference disclose every element of the claimed invention and also a motivation to combine the references. With regard to the example above, a rejection under 103 is proper if the Office can show that all of the elements A, B, and C, and the motivation are disclosed in at least two prior art references. If the Patent Office makes the requisite showing, applicants can submit evidence of unobviousness, such as unexpected results, commercial success, long felt but unresolved need, copying by competitors, etc.
The requirements for patentability are discussed in MPEP chapter 700.