Amplifier: an electrical device that increases the magnitude and power of a signal before it is delivered to the transducer.
Current: a flow of electrons is called an electrical current, and represents the ability to generate power.
Distortion: additional, spurious signals that occur when a transducer is driven too hard. Distortion is generally considered a negative artifact.
Frequency: how often the signal changes per second. The lower the number, the lower the tone (bass tones). High numbers are very bright (like a high pitched whistle).
IEM: In- Ear Monitor. A headphone designed to reside mainly inside the ear structure and directly couple the output of the headphone into the ear canal.
Impedance: a measure of how an item resists a flow of current for a given voltage. Voltage, current, and impedance are related by Ohm's Law (V = I * R). Impedance can vary with frequency.
SPL: Sound Pressure Level. A measure of the relative volume of a sound. Higher numbers mean a louder sound.
Transducer: a small motor that generates SPL based upon an input signal from an amplifier.
Voltage: a measure of the electric potential of a signal. As this signal is placed across an impedance, a current flows. If current flow is not sufficient, then the voltage will drop down and distortion will result.