A fact is a thing that is known to be consistent with objective reality and can be proven to be true with evidence. For example, "this sentence contains words" is a linguistic fact, and "the sun is a star" is a cosmological fact. Further, "Abraham Lincoln was the 16th President of the United States" and "Abraham Lincoln was assassinated" are also both facts, of the historical type. All of these statements have the epistemic quality of being "ontologically superior" to opinion or interpretation — they are either categorically necessary or supported by adequate historical documentation.
Conversely, while it may be both consistent and true that "most cats are cute", it's not a fact (although in cases of opinion there is an argument for the acceptance of popular opinion as a statement of common wisdom, particularly if ascertained by scientific polling). Generally speaking, facts transcend belief and serve as concrete descriptions of a state of affairs on which beliefs can later be assigned.
The usual test for a statement of fact is verifiability — that is whether it can be demonstrated to correspond to experience. Standard reference works are often used to check facts. Scientific facts are verified by repeatable careful observation or measurement by experiments or other means.