A circular particle accelerator. A synchrotron is a versatile accelerator and can accelerate various charged particles, such as electrons, protons and even heavy ions to very high energies. Most of today's physics research accelerators are synchrotrons, with the notable exception of SLAC. Storage rings are a kind of synchrotrons. At CERN and Fermilab there are several synchrotrons/storage rings.
The particle-path is ring-shaped (or square with rounded corners) with a constant diameter. The diameter is held constant by increasing the field strength in the bending magnets as the energy increases. Because of the constant diameter the whole accelerator need not be inside a magnetic field, as in a cyclotron. Only small (or rather large, actually) magnets are needed in the corners. Between the corners with magnets are straight sections with accelerating cavities, magnetic quadrupole lenses for beam focussing and injection/extraction devices. Experiments are sometimes conducted directly in the beam path, especially in storage rings.
In a synchrotron the particles are accelerated by a microwave/UHF field in a resonator cavity. In order to keep up with the increasing particle speed, the frequency must increase at the same time, so as to keep the particle beam and field synchronous, hence the name synchrotron. By using a pre-accelerator (for example, a linear accelerator or a cyclotron) this frequency swing can be kept reasonably small. Since electrons are very light, it is easy to get them to relativistic velocities and the frequency can be kept constant.
For very high energies the losses due to synchrotron radiation in a electron synchrotron gets very high and the radius must increase to keep the curvature low. LEP at CERN has a maximum energy of 100 GeV and a circumference of 27 km, still the losses are huge. The total voltage needed over the accelerating gaps to keep up with the losses are 3 GV! Circular electron-positron colliders with higher energies are impractical and future plans are to use linear accelerators instead, as at SLAC. For heavier particles the problem is that the field strength in the magnets gets very high and superconducting coils are needed.