Pharmacology is the study of how drugs affect living organisms; it is comprised of pharmacodynamics, which deals with how drugs act on receptors and other biologic targets (like enzymes) within the organism in question (for our purposes this is, of course, humans) to produce their respective effects and pharmacokinetics which is concerned with how the body interacts with the drug, like how the drug is absorbed, distributed, metabolized and then excreted by the body.
Pharmacodynamics is usually quantified using three distinct measurements: binding affinity (towards a receptor or enzyme of biological importance; often abbreviated Ki), intrinsic activity (towards a receptor only; abbreviated IA) and half maximal inhibitory or effective concentration (IC50, EC50, respectively). Binding affinities refer to how potently a drug, hormone or toxin binds to its receptor/enzyme target. Intrinsic activities refer to how completely a drug can activate a receptor (measured as a % of the extent that the endogenous ligand activates said receptor). EC50/IC50 refers to the concentration of the drug required to activate or antagonize a physiologic response (like receptor activation or receptor activity) by 50%.
Pharmacokinetics and its details are listed here.