In the field of patents and intellectual property, the state of the art has a significant meaning, and can cause infuriation to those who are trying to apply for a patent. The definition, to quote from the UK's Patent Act, is:
“The state of the art in the case of an invention shall be taken to to comprise all matter (whether a product, a process, information about either, or anything else) which has at any time before the priority date of the invention been made available to the public (whether in the UK or elsewhere) by written or oral description, by use or in any other way”
The priority date of a patent is the date at which a patent has first been filed in a patent office. A patent may claim priority from this application if filed in another office, with a cut off point of a year's difference. You could, for example, file a patent in the US, and then, six months later, file the same invention in the UK. The priority date of the invention will therefore be the date you first filed, which was in the US.
In order for your patent to be granted it must be novel - so it must be claimed in such a way that the claim doesn't describe anything that has been public before the priority date. Often inventors are unaware of a great deal of prior art, and are frustrated by the fact that their invention, although not available in the shops, has been thought of before, and is therefore unpatentable.
There is, as well as novelty, the hurdle of inventive step (sometimes called obviousness) that must be overcome before a patent is granted. To deal with such questions, the courts have invented an individual known as the Man Skilled In The Art. He is a rather odd guy, since he knows everything about the prior art in his particular field of technology. He may even be a team of Men Skilled In The Art. But this Man has one thing he can't do – he can't make inventive leaps over the prior art. He can only make obvious advancements over what is known, and in many cases combine the knowledge gained from different sources to solve a problem. But, strictly, he can't invent.
To be inventive, the invention, as claimed, must not be obvious to this Man Skilled In The Art. If he, being uninventive, would never come up with the invention, then the second barrier to getting a patent has been overcome.
There are other barriers to gaining a patent – and the most prominent one at the moment is that computer programs, amongst other 'non-technical' fields cannot be patented. The question of prior art is that is it actually possible for a computer program to be inventive, given that it's just the manipulation of known commands and processes? Those who vigorously oppose software patents argue that anything that relates to software is completely unpatentable for this (and other) reasons. Others disagree, and say that certain kinds of software related inventions can actually be technical in nature, and therefore patentable. This is an ongoing debate, and one that will not be over quickly.