Here, we will explore how electric current works. Electric current represents the flow of electrical charge in a conductor and it's measured in amperes, symbolized by A. Voltage represents the difference in electrical potential between two points of a circuit. It is measured in volts, symbolized by V. Let's think of a battery. Each battery can be considered a voltage source, and it has two terminals, a positive (+) and a negative (-). Following is one of the standardized symbols for voltage sources:

In the diagram, we see a voltage source that produces 9 volts. This means that the positive terminal has a 9 V difference over the negative terminal. The negative terminal is usually referred to as ground, GND for short. An important convention when dealing with current is the direction of current flow—from higher potential (voltage) to lower. The following diagram shows how the current flows from the positive terminal, through a resistor, back to the negative terminal:

Resistance is the measure of the property of a material to oppose current flow. It's measured in ohms, symbolized by the Greek letter Ω. The resistor is the component that uses its internal resistance to restrict current flow. This is the schematic symbol, and next to it, a normal resistor:

In the following section, we will see how resistors function in a circuit.