Nuclear spins in a constant magnetic field will "flip" if one applies a magnetic field perpendicular, RadioFrequency field. This field has to be at the right frequency. As the spacing of the energy levels for spin 1/2 particles(we are looking at protons) depends on the applied magnetic field. Something like \hbar \nu = 2mB, where m is the proton mass. By applying the RF field we "flip" some of the protons with spin parallel to the static B field into the antiparallel direction(this has higher energy). As all systems the protons will "decay"/relax back into the lower energy configuration. This relaxation will induce a RF signal which we can pick up(XXX what use is it?)
Larmor frequency explained with nice pictures. The semi-classical idea of tops precessing around the static magnetic field is helpful, sometimes.
Relaxation times explained and given for various bits of tissue in the human body.
On the first day in lab we did Continouse wave NMR stuff. we found the resonance freq, but as we only had a permanent magnet we could not vary field strength to see how frequency of the RF field varies with field strength.