FRET means Fluorescence with Resonance Energy Transfer. We (that is, George Coker, Simon Chen and myself, Wieb van der Meer) have written a book about it (see publications) except we called it RET then (Resonance Energy Transfer) because at that time, 1994, there were 3 competing abbreviations: FET, RET and FRET. We bet on the wrong horse. Also our publisher (VCH) was eaten by the another one: Wiley, and our book was no longer promoted. However, it is still available. It is outdated though, because after 1994 the FRET field underwent a new explosion: FRET microscopy. But, first let me explain what FRET really is: it is what Stryer called a spectroscopic ruler. It is a technique to measure distances between 1 and 10 nanometer by examining the emission spectrum of a "Donor". The Donor is a molecule that can absorb light and can also fluoresce. An "Acceptor" is also present. An Acceptor is a molecule that can absorb light. It does not have to be fluorescent (but it helps). If the Donor and Acceptor are close to each other on a relative scale (see below) then the probability of transfer is high: first, the Donor absorbs light and enters an excited state in which its energy is higher than normal (higher than in the "ground state"). When an Acceptor is present 1 of 2 things can happen:
          one, the Donor can emit fluorescent light and return to the ground state. This is called Donor emission. Second, the Donor can undergo a resonance interaction with the Acceptor and the Acceptor can pick up a "virtual" photon. Therefore, when an Acceptor is present the intensity of the Donor fluorescence is less than when the Acceptor is absent. Foerster, the genius who discovered FRET in 1948, showed that the FRET efficiency (=1 minus IDA/ID, IDA is the Donor emission in the presence of Acceptor, ID is that in the absence of Acceptor) is equal to: Foerster distance^6/(Foerster distance^6 + distance^6). The Foerster distance depends on some relatively easily measured things and on an orientation factor which cannot be directly measured. I am an expert on this orientation factor.