IC50 is a measure of the concentration required to give 50% of the maximal biological effect on the target. Where an inhibitor is administered at IC50, the activity of the target enzyme will be half Vmax where no inhibitor is present.
EC50 is the effective concentration 50%, giving 50% activation of the target. This is a very similar measure to IC50, but is for the opposite effect.
The potency of an enzyme inhibitor can be determined by the dose required to obtain IC50. A lower concentration to get to IC50 indicates a better, more potent inhibitor. This allows comparison between different inhibitors for the same target enzyme.