Islands are land areas smaller than a continent that are completely surrounded by water. Islands range in size from islets (small islands) barely exposed at high tide, to vast landmasses almost the size of continents. Islands exist in all the ocean basins (the deep part of the ocean floor), along coastlines, and in freshwater lakes and rivers. Islands come in many sizes and shapes, but they all share the same defining characteristics. There are more similarities than differences between a huge arctic island like Greenland and a small tropical one like Guam.

Islands are isolated. The water around them controls their climate and weather. The British Isles, which include Great Britain and Ireland, have a mild climate for their northern location because they lie in the path of the warm Gulf Stream current (a current is a steady flow of water on a prevailing direction). The Galapagos Islands...