Markram, H. Lubke, J. Frotscher, M. Sakmann, B. (1997) Regulation of Synaptic Efficacy by Coincidence of Postsynaptic APs and EPSPs. Science 275, 213.
Bi, G. Poo, M. (1998) Synaptic Modifications in Cultured Hippocampal Neurons: Dependence on Spike Timing, Synaptic Strength, and Postsynaptic Cell Type. The Journal of Neuroscience 18(24): 10464-10472.
These were the first two papers that showed STDP experimentally. STDP is a learning rule that is dependent on the precise temporal order of spiking in the pre- and post-synaptic cells. Bi, Poo show it nicely:
There's extensive work on the mechanisms of STDP. The primary mechanism is an NMDA channel, I'll explain that in more detail later. The signal is based on Calcium - excess calcium causes a chemical cascade that recruits or removes AMPA receptors to the synapse.
These papers naively think about the neuron as a single compartment. There is much more work about how STDP is really dependent on the dendritic spike - this is a big calcium signal. Eliciting an action-potential (or burst which is sometimes necessary to get this effect), will influence the dendrites and make a dendritic spike more likely. So the learning rule is not really based on pre-post spikes, but whether the dendrite spikes.
There are a lot of other factors that can modulate this learning rule - frequency, synaptic strength, chemical modulators. Dopamine is an especially interesting one as it has been implicated in reinforcement learning and has been shown to be able to modulate STDP.