Any area of land smaller than a continent and entirely surrounded by water. Islands may occur in oceans, seas, lakes, or rivers. A group of islands is called an archipelago. Islands may be classified as either continental or oceanic. Oceanic islands are those that rise to the surface from the floors of the ocean basins. Continental islands are simply unsubmerged parts of the continental shelf that are entirely surrounded by water. Many of the larger islands of the world are of the continental type. The islands that rise from the floors of the ocean basins are volcanic. Lava accumulates to enormous thickness until it finally protrudes above the ocean surface.
Source: Taken from Encyclopaedia Britannica 2002