Normally volcanoes form at plate boundaries, but hot spots are an exception. Hot spots are areas in the mantle that are exceptionally hot, and form plumes that break through the crust. They are great evidence of plate tectonic movement because they form island chains. Take Hawaii for example- the big island of Hawaii is directly above a hot spot and is still actively growing. The further you get away from the hot spot, the older the islands are. That means Kauai is the oldest of the chain.
For a long time I was looking for a way to demonstrate to students how hot spots work. Here is a great demo you can use that only requires a few materials: an aluminum pie pan, cornstarch, water, and a candle. Check out the video to see it in action: