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A hotspot is a region of high volcanic activity not directly connected to a tectonic plate boundary. It is caused by upwelling of deep mantle plumes. Since their locations within the Earth are relatively "fixed", slow tectonic plate movements over them can create volcanic island chains/undersea ridges. Geologists have identified some 40-50 such hotspots around the globe although the causes of volcanism at some of them is debated. See this site for list and map. Recently it is understood that they are no longer 'fixed' as previously thought due to the recent study published in Science magazine and reported on here at Science Daily. Hotspots are not stationary they move because the Earth's mantle is in constant motion. See Science Daily
