A circuit is the path of electric flow. This means the path or way the electrons move from the negative part of the battery to the positive part of the battery.

circuitbjw2.jpg
This circuit has four parts:
  • the D-cell battery
  • the switch
  • the light bulb
  • the wire
When the switch is on, the circuit is complete and electrons move from the negative to the positive part of the battery because there are less electrons in the positive side.
When the switch is off, the circuit is incomplete. This means there is a connection breech, or a gap in the circuit that won't allow the electricity to flow through the entire circuit.

picture from:
http://www.myschoolhouse.com