Automatic writing - sometimes called Spirit Writing - is the ability to write down (or type, this is the 21st century!) your thoughts in a semi-conscious manner to produce a free flow of text that can be re-read afterwards. Proponents claim variously that this is a link with God (see the book 'Conversations With God' by Neal Donald Walsch), spirit guides, our 'inner soul' or even dead people, but they often claim that the writings come from 'outside' of the writer, that they are somehow merely the conduit for something else. Some, more scientifically minded scientists, purport that writers are in a dissassociative state and their subconscious mind is what is driving the pen (or typing the keys).
Some famous texts are claimed to have been written by automatic writing. David Icke claims that his book Son of the Godhead came from the practice, as did Aleister Crowley's The Book of the Law. After Dickens died his unfinished The Mystery of Edwin Drood was 'completed' by publisher T.P. James who claimed it was written by the spirit of Dickens channeled through a spiritualist summoning (and was available at all good bookshops). The Book of Mormon as written by Joseph Smith is often said to be automatic writing because he claimed it was dictated to him by the Angel Moroni. André Breton, one of the founders of the surrealist movement in mid 19th C France wrote several books, including The Surrealist Manifesto, using the technique.
Of course the fatal flaw and all of these claims is that there is no way to know whether the writer is really in an unconscious state or is simply pretending to be. It is not unreasonable that we can almost switch off when writing things that we are trying to get out onto paper. Our thoughts move much quicker than our hands and so we often know what we want to say before it is written down. But whatever is written comes from within us. Any claim that the information comes from an external, independent source can only be judged on what is actually written, and to date the evidence for that is missing.