Basic DC Circuits:
In a DC circuit, power flows in one direction: from the positive side of the power source, through the load, to the negative side of the power source.
To switch the load, you can place the switch either in front of the load (on the positive leg of the circuit) or behind the load, on the negative leg.
Sinking Switch Circuit
Sinking is switching the load in the negative leg of the circuit, as shown here.
Sourcing Switch Circuit
Sourcing is switching the load in the positive leg of the circuit, as shown here.