The Van Allen belts, also referred to as the Van Allen radiation belts, are two large toruses of charged particles around the planet, held in place by the Earth's magnetic field. The Van Allen belts exist because of "blind spots" in the Earth's magnetic field caused by its compression and stretching from solar wind.
The Earth's magnetic field serves as a magnetic mirror, bouncing charged particles back and forth along force lines stretching between the North and South Magnetic Poles. The Van Allen belts are closely associated with the aurora borealis and the aurora australis, beautiful curtains of charged particles visible in locations on the Earth's surface where the Van Allen belts intersect with the upper atmosphere. The Van Allen belts are also relevant to satellites and orbiting space stations, which have to avoid the belts due to the damage that their charged particles would cause.
In the late 19th century and early 20th century, several scientists -- Carl Størmer, Kristian Birkeland, and Nicholas Christofilos -- speculated on the possibility of a belt of charged particles surrounding the Earth, but it wasn't until 1958 when its existence was confirmed by some of the earliest American satellites, Explorer 1 and Explorer 3. The projects were led by Dr. James Van Allen of the University of Iowa, after which the belts were named. Explorer 1, a 14 kg (30 lb) space probe, was launched for the International Geophysical Year, and the scientific data it returned about the space immediately outside of Earth's atmosphere was invaluable. The Van Allen belts were initially discovered when the cosmic ray detection equipment on the satellites went temporarily dead, overwhelmed by the local radiation.
There are two distinct Van Allen belts -- the inner Van Allen belt and the outer Van Allen belt. The inner Van Allen belt, extending from 0.1 to 1.5 Earth radii from the surface, consists of highly charged protons, capable of penetrating up to a millimeter of lead and doing damage to astronauts and space equipment. The outer Van Allen belt, located between 3 and 10 Earth radii from the surface, with its greatest intensity between 4 and 5 Earth radii out, consists of energetic electrons. The source of the energetic particles varies depending on the belt -- the inner Van Allen belts consist of decay products from cosmic ray impacts with the upper atmosphere, while the outer Van Allen belts are produced from influxes of charged particles from geomagnetic storms, which are themselves produced by the influence of the Sun on the Earth's magnetic fields.