* Noun: This is the most common usage, referring to a ridge of rock, coral, or sand just below the surface of the water. Example: "The coral reef was teeming with colorful fish."
* Verb: This is less common, but it can mean to form a reef or to obstruct something. Example: "The rocks reefed the boat's propeller."
So, without more context, it's hard to definitively say which part of speech "reef" is.