Voltage and current
The speed of the waterwheel changes by changing the water volume in the tank. This means that the speed of the water flowing to the waterwheel changes with the change in the water pressure in the tank.
When this phenomenon of water is substituted with electricity, the water volume (water pressure) is the voltage, and the water flow is the electrical current.
Current and resistance
The force of the water flow changes with the height of the gate that is located between the tank and the waterwheel. As a result, the speed of the waterwheel changes.
This gate is equivalent to the resistance in an electrical circuit. Read more Basic of Electrical Circuit