FRET, or Fluorescence Resonance Energy Transfer, is a method by which molecular and cellular biologists can determine the close physical proximity of two molecules.
Fluorescent molecules are fluorescent because, when excited by a certain wavelength of light (an excititation wavelength, specific to each fluorescent molecule), emit light at a different wavelength (emission wavelength; again, specific to the molecule). Basically, the light hits the molecule, boosts the energy of the electrons on the molecule, and as the electron drops back to the lower energy state, emits a photon. This process isn't perfectly efficient, and the molecule can bleed off energy in other ways besides emitting a photon.
It just so happens that one of these other ways involves transmitting the energy to another, nearby fluorescent molecule. So let's say you have fluorescent molecule 1 and 2, each with distinct excitation and emission wavelengths. Normally, hitting molecule 1 with it's excitation wavelength gives you back molecule 1's emission wavelength. Same for molecule 2. But if you have the two molecules close enough, when you hit the pair with molecule 1's excitation wavelength, you get some of molecule 2's emission wavelength back (with a corresponding decrease in molecule 1's emission wavelength brightness).
It just so happens that you get this effect in the range of 10 Angstroms, which is pretty much touching each other as far as biological molecules go. This physical phenomenon has been utilized by researchers to prove the direct physical interaction of a number of biologically important molecules.
You can do FRET-type experiments both with a fluorescence microscope, as well as with a flow cytometer (FACS).