Figure 1   Resistors are labeled using colored bands
For example, consider the resistor shown in Figure 1. The color bands are brown, black, red, silver (1 0 2 10%). This is a 1000 Ohm resistor with 10% tolerance.
Color Number Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Gray 8 White 9
Table 1   Resistor color codes
Ohm's law states that the voltage across a resistor is proportional to the current flowing through the resistor.
Ohm's law does not hold in all situations. Resistors are designed using linear materials, so that resistors follow Ohm's law. Other useful devices such as diodes and transistors are nonlinear. They do not obey Ohm's law.
Take a series of measurements. Plot the current as a function of voltage.
Figure 1   A circuit to verify Ohm's law is shown. Current flows through the ammeter and through the resistor. The voltmeter has a high input resistance and draws very little current. It measures the voltage across the resistor.
Figure 2   Resistors in series.
Figure 3   Resistors in parallel.
Figure 3   Resistors in series and in parallel.