Prior art is anything that is older than your invention's conception date. So prior art really relates to anything. It could be magazine articles or academic theses that are in a library somewhere, or it could include prior publications in a journal. It could include your own inventions that you have made in the past as well as other patents that are owned by other parties and other companies. Anything that predates your invention date that relates and is a material to the patentability of your invention could be considered prior art. In short, anything that is close or related to the patentability of your invention can be prior art.
As you may have already guessed, prior art is a very broad term. It is really used to refer to any inventions prior to yours as well as prior disclosures of those inventions to the world prior to yours by others or potentially yourself if you did not previously disclose them to the Patent Office. In the United States, what determines whether something counts as prior art is based on the invention date – not the date you filed but the date you have substantiated proof that you invented something first.
It is important to remember that patent prior art and also trade publications can include things that are in different languages and can come from anywhere in the world. So if something was printed in Japanese and disclosed your same concept two years before you came up with it, the fact that it was in Japanese would not affect its status with regard to having created the concept ahead of you. It is still prior art against you, which is to say that the published Japanese statements prove that you were not the first to invent your concept.
If it was common knowledge in the to the world, it is also considered prior art in the form of prior knowledge. There is a statute referenced as, 35 U.S.C. 102, which lists many different instances that could qualify as a potential basis of rejecting your claims on the basis of novelty and evaluating that could help determine what classifies as prior art and what does not. Essentially, your invention date and whether things were published by others before that invention date, and whether there are other inventions that you made prior to invention date that are material to patentability of your current invention, are referenced as prior art.