A known known is information that is fully studied and well understood, so that an individual or organization can have confidence in its comprehension and relevance. In project management, for example, capitalizing on known knowns can help reduce risk.
A known unknown is information that is understood to exist but is not in the possession of the person seeking it. For example, someone querying a search engine might be looking for the time of a movie: They are aware that the movie is scheduled for a particular time but do not know what that time is.
An unknown known is information that an individual or organization has in its possession but whose existence, relevance or value has not been realized.
An unknown unknown, the fourth category, is information that a person or organization is completely unaware of. Unknown unknowns represent the most risk to a business because something that is unforeseeable is very hard to prepare for.