The effects of caffeine (and all drugs) are dose/concentration-dependent. At the doses typically consumed by humans, caffeine is primarily an adenosine receptor antagonist. However, higher doses of caffeine can inhibit phosphodiesterases, release intracellular calcium, block GABAa receptors, and have a number of other effects. Also, adenosine receptors interact with receptors from other neurotransmitter systems and modulate their function (e.g. NMDA and dopamine receptors). There are many good reviews on this topic (check out reviews by Drs. Bertil Fredholm, Sergi Ferré, Joaquim Ribeiro, JW Daly and Per Svenningsson).