An ampere (or amp) is a measure of the amount of electricity, called "current," in a circuit, while voltage is a measure of the force behind that electricity's motion. In a common textbook analogy in which a circuit is imagined as a garden hose, current (measured in amps) would be the volume of water within the hose and voltage would be the pressure that pushes it onward.
Other units of measurement further define the relationship between voltage and current: resistance and wattage. Measured in ohms, resistance in a circuit directly limits the amount of current produced by voltage. Thus, current is equal to voltage divided by resistance. Wattage is the total electrical power produced by the circuit and is equal to current times voltage.