Small changes in gate potential produce large changes in the current flowing from source to drain.
Here is a water analogy of a MOSFET. Pressure on the hose depletes the available water under your foot so the flow is cut off. The gate in the diagram below is your foot.
Field effect transistors are three pin devices. The pins are called the SOURCE, GATE and DRAIN. (Don't confuse these voltage controlled devices with bipolar transistors which are current controlled.)
This diagram shows an N Channel MOSFET. The insulation between the gate and the channel is very thin and easily destroyed by electrostatic discharge. Anti-static precautions need to be taken when handling MOSFETS.
The gate potential causes an electric field in the gate region of the MOSFET. This potential alters the number of N Type carriers in the gate region.
ENHANCEMENT: A positive gate potential attracts more carriers so more are available so more current can flow.
DEPLETION: A negative gate potential repells the carriers so fewer are available and less current flows.
P Channel MOSFETS are available too. They work in the same way but a negative potential causes enhancement.