An atoll is formed from an island, but it isn't entirely an island. It's a coral reef that makes the different. The reef grows in a ring around the island and the island erodes, until little if any of it is above the surface of the water and the coral reef remains. Islands are any small body of land surrounded by water. Islands in the middle of the ocean are usually volcanic and islands in lakes and near continents are usually just areas of the continental shelf surrounding the continent with a higher elevation. To actually answer your question, the KEY difference is the reef.