Stop signs are octagons. An octagon is a shape that has eight sides, whereas a hexagon only has six sides.
In North America, you will find stop signs to assist right of way for pedestrians and traffic. They are used in areas of towns and cities that are not large enough to warrant the use of traffic lights. Stop signs can be used alone or as part of a three-way or four-way stop.
Both hexagons and octagons can be irregular or regular. A regular shape has sides that are all equal in length. An irregular shape has sides of different lengths.