Difference between NMR and ESR

NMR = Nuclear Magnetic Resonance is the resonance that occurs when a nucleus (usually hydrogen, but any nucleus that has non-zero spin will work) is placed in a magnetic field and is 'swept' by a radio frequency that causes the nuclei to 'flip'. This causes the radio frequency to be absorbed, which is what is measured. 


ESR = Electron Spin Resonance is also a resonance phenomenon, except in this case it is the spin of an unpaired electron that is in resonance, rather than a nuclear spin.