Leaves are flat organs of plants that stick out to catch sun to perform photosynthesis. They're typically green because of the chlorophyllic chemicals used in the photosynthesis process. You really should know what a leaf is.
They're not usually designed to store energy, so they usually don't taste great, at least in my opinion. However, this does mean that they are very low in calories, and they are high in fiber, so some people eat them despite their taste.
Culinarily, leaves are always vegetables. They do not contain seeds and are nothing like botanical fruits. They're also not sweet, so I don't know why someone would think they could be fruits culinarily anyway.