The former is the strengthening of the synapse between two neurons if the postsynaptic spike temporally follows immediately after the presynaptic spike. Latter is the case if it is reverse, i.e., the presynaptic spike occurs after the postsynaptic spike. Gerstner, W. & Kistler, W. (2002)[3] in Chapter 10 and Sterratt, D., Graham, B., Gillies, A., & Willshaw, D. (2011)[2] in Chapter 7 discuss the various models related to Hebbian models on plasticity and coding.