A white paper is a document that has one main purpose, to persuade potential investors how great the project is. It does not function as a manual or other technical material but rather as a marketing material. That being said, a white paper differs from typical marketing collateral such as brochures because it is rather a report that presents a problem and provides a solution, written in an academic style and in a convincing way. A white paper is based on actual facts and evidence.
See how a white paper can look like at https://bitcoin.org/bitcoin.pdf