A brief summary of the inhibition / excitation theory of extinction and spontaneous recovery in classical conditioning (also discussed in Mazur on p. 70-71):

1) An excitatory CS-US connection builds up to its maximum strength during the acquisition phase of classical conditioning (through contiguity of CS and US, for Pavlov), so that the CS alone will produce the CR.
2) During the extinction phase (CS presented with no US), an inhibitory CS-US connection builds up, tending to prevent the CR -- as if the animal learns that the CS does not lead to the US after all.
3) When the strength of the inhibitory CS-US connection becomes equal to the strength of the excitatory connection, extinction is complete and no CR occurs.
4) During a rest period, the inhibitory CS-US connection weakens and disappears, but the excitatory connection does not.
5) When the CS is next presented after the rest period, inhibition has almost completely dissipated, so the excitatory connection can again produces the CR: this is spontaneous recovery of the CR, at a slightly lower strength than it was initially.
6) With each succeeding extinction, the inhibitory CS-US connection becomes more permanent (i.e., it weakens less each time) -- so eventually it loses NO strength during the rest period, and no spontaneous recovery happens.