When electrons (or any charged particles) are accelerated to keep them in a circular path they will radiate electromagnetic radiation in a narrow beam in the direction that they were travelling. This radiation is called synchrotron radiation. ("Synchrotron" is the name given to any circular accelerator that uses microwave electric fields for acceleration and magnets for steering.)
For high-energy physicists the synchrotron radiation is a nuisance because the electrons lose energy and must be reaccelerated to keep them travelling around the ring. Otherwise the match between their momentum, the magnetic field and the radius of the ring is quickly lost. As the particles approach close to the speed of light the amount of energy lost to synchrotron radiation for a fixed radius ring grows rapidly (as gamma = (1-(v/c)**2)**(-1/2)). This means that one needs to make bigger radius rings to store higher energy particles and provides a practical limit to the energy that can be reached in a synchrotron. (Note that the limit is quite different for protons than for electrons, since at a given energy electrons have a much larger value of gamma than protons (E = gamma m c**2 where m is the (rest) mass of the particle.)
However, at the energies of the SPEAR storage ring at SLAC, the synchrotron radiation turns out to give a beautiful beam of x-rays which another group of physicists have exploited as a tool to study many aspects of the structure of matter at the atomic and molecular scale, from surface properties of semiconductor materials to the structure of protein molecules. This is the work of the division of SLAC known as the Stanford Synchrotron Radiation Laboratory (SSRL).
Originally this work began as a "parasite" project, using whatever synchrotron radiation was produced during high-energy physics use of the storage ring. Over time so much interesting research developed that SSRL has now completely taken over the SPEAR storage ring and built a new low-energy (3 GeV) accelerator so that SPEAR can be filled with electrons even when the main SLAC accelerator is busy doing other research. Additional magnets known as "wigglers" have been added in straight sections of the SPEAR ring to produce even more intense x-ray beams due to radiation as electrons "wiggle" through the alternating sections of magnetic field from these magnets. Some of the research areas studied with synchrotron radiation x-rays are discussed below.