To qualify for a patent, an invention must be nonobvious as well as novel. An invention is considered nonobvious if someone who is skilled in the particular field of the invention would view it as an unexpected or surprising development.
For example, in August 2013, Future Enterprises invents a portable, high-quality, virtual reality system that can be manufactured for under $100. A virtual reality engineer would most likely find this invention to be truly surprising and unexpected. Even though increased portability of a computer-based technology is always expected in the broad sense, the specific way in which the portability is accomplished by this invention would be a breakthrough in the field, and thus unobvious. Contrast this with a bicycle developer who uses a new, light-but-strong metal alloy to build his bicycles. Most people skilled in the art of bicycle manufacturing would consider the use of the new alloy in the bicycle to be obvious, given that lightness of weight is a desirable aspect of high-quality bicycles.
Knowing whether an invention will be considered nonobvious by the USPTO is difficult because it is such a subjective exercise -- what one patent examiner considers surprising, another may not. In addition, the examiner will usually be asked to make the nonobviousness determination well after the date of the invention, because of delays inherent in the patent process. The danger of this type of retroactive assessment is that the examiner may unconsciously be affected by the intervening technical improvements. To avoid this, the examiner generally relies only on the prior art references (documents describing previous inventions) that existed as of the date of invention.
As an example, assume that in 2015, Future Enterprises' application for a patent on the 2013 invention is being examined in the Patent and Trademark Office. Assume further that by 2015, you can find a portable virtual reality unit in any consumer electronics store for under $100. The patent examiner will have to go back to the time of the invention to fully appreciate how surprising and unexpected it was when it was first conceived, and ignore the fact that in 2015 the technology of the invention is very common.