A particle accelerator is a device that provides a high-energy beam of elementary particles. This includes many devices that do not fit the general conception of a 'particle accelerator' as a large machine that works at insanely high energies. The first particle accelerator was the Crookes' Tube, ancestor of the modern Cathode Ray Tube, so your television is in fact a particle accelerator (unless you have one of those fancy plasma TVs). The first particle accelerator which fit the modern stereotype of a particle accelerator was Ernest Lawrence's cyclotron.
Early accelerators such as the Crookes' Tube and Van de Graaff generator accelerated only electrons, but the cyclotron was sufficiently general to accelerate any charged particle. It also permitted much higher energies than any previous accelerator design. The invention of the cyclotron and the uses to which it was immediatly put earned Lawrence the 1939 Nobel Prize for Physics.
There are three major types of particle accelerator in research use today. One of those is the venerable cyclotron which uses a constant magnetic field and an alternating electric field in a disc-shaped chamber. The second and most prominent is the synchrotron which uses an increasing magnetic field in a ring-shaped chamber. Finally, the third form of accelerator is the linac, or linear accelerator, which is essentially a scaled-up CRT, with the particle passing through a set of large potential differences. All three of these devices can concievably be used with any charged particle, although specific implementations are limited to specific species of particle. The largest synchotron in the world is currently the Relativistic Heavy Ion Collider at Brookhaven, although the LHC at CERN will unseat it after it comes on line. SLAC houses the largest linac in the world, and the largest cyclotron is found at the Canadian lab TRIUMF.
In physics research, accelerators are generally used for collision experiments. The large synchrotrons accelerate two opposing beams of opposite charge and then cross them in the heart of a detector. Smaller synchrotrons, linacs, and cyclotrons direct the particle beam onto stationary targets. Often these targets are not used for experiments but are used to produce short-lived secondary particles such as pions and muons. These secondary particles are then gathered into a beam and collided with an experimental target in a detector. These 'production targets' are also being used with proton beams to produce unstable isotopes of heavy elements ('heavy' in this context essentially means anything but hydrogen and helium).
Particle accelerators also have a place outside physics research. Cyclotron-produced radioisotopes are used in medical imaging, including PET. Relatively large linacs are used in high-intensity X-ray generators for cancer therapy. (see also proton beam therapy) Muon beams can be used to probe the structure of materials, which is useful not only in solid state physics but also in mechanical engineering. Transuranium elements can be created in order to study their chemistry. Still, the largest research usage of particle accelerators is in physics, in particle, nuclear, atomic and solid-state physics experiments.
This writeup is copyright 2002 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at http://creativecommons.org/licenses/by-nd-nc/2.0/ .