The Simple Circuit

A simple circuit contains the minimum things needed to have a functioning electric circuit. A simple circuit requires three (3) things:

A source of electrical potential difference or voltage. (typically a battery or electrical outlet)
A conductive path which would allow for the movement of charges. (typically made of wire)
An electrical resistance (resistor) which is loosely defined as any object that uses electricity to do work. (a light bulb, electric motor, heating element, speaker, etc.)

Despite being considered simple, they are useful and quite common. They exist as flashlights, doorbells, and many kitchen appliances. From a Physics point of view they display many of the rules that govern the behavior of electricity in circuits. Below is an animation of a simple circuit where electrical energy is shown as gravitational potential energy (GPE).

In this animation you should notice the following things:

The battery or source is represented by an escalator which raises charges to a higher level of energy.
As the charges move through the resistor (represented by the paddle wheel) they do work on the resistor and as a result, they lose energy.
By the time each charge makes it back to the battery, it has lost all the energy given to it by the battery.
As the charges move through a wire, they lose no energy (assuming the wires are short and are a good conductor).
The potential drop ( - potential difference) across the resistor is the same as the potential rise ( + potential difference) across the battery. This demonstrates that a charge can only do as much work as was done on it by the battery.
The charges are positive so this is a representation of Conventional Current (the apparent flow of positive charges)
The charges are only flowing in one direction so this would be considered direct current ( DC ).

Ohm's Law describes mathematically the relationship between current and voltage (potential difference). The more potential difference you have the greater your current is going to be. The more resistance a circuit has, the lower the current is going to be. The following equation is Ohm's Law. It holds true for any circuit as long as temperature does not change.