The term 'prior art' has now been firmly ingrained into the patent community vernacular. Prior art is the collection of all things publically known which relate to the invention. The prior art is made up of patents, and documents which are not patents. Every document in every library – and even things which are not documents, for example a video, may be considered 'prior art'. That is a large body indeed. In short, any public disclosure of any type might be considered prior art for purposes of patent application examination.
Most often, one considers prior issued patents and sometimes published patent applications – as prior art. This is due to the fact that a patent examiner generally starts in the patent files of over 8 million patents (Pat. No. 8,000,000; August 16, 2011) to build a case against patentability for an application under examination. While a typical examination only involves US and sometimes foreign patents, there is nothing which would prevent an examiner from considering other sources. Indeed, professional technical journals contain a very formidable body of unique work and these might be considered an even more potent source from which patentability may be considered. However, patent examiners like most everyone else tend to rely on that which is near and easy. It takes considerable effort to propertly search and digest material from these highly specialized and variable technical publications.
A patent examination depends upon the prior art because – an invention must new and non-obvious. It is against this background, the prior art, from which such comparison is made. So, a patent examiner will gather all things (patents) most similar to an invention under examination and apply tests for anticipation (novelty) and Obviousness.aspx">obviousness. After applying these tests, the examiner will form rejections to the patent claims – in view of the prior art.