Resistors.

What is a resistor?
A resistor is an electronic component which is designed to oppose the flow of current. The higher the resistance value, the more it will oppose current flow and the lower the current through the resistor will be.

Values
Resistors come in a very wide range of values, from less than 1 Ohm, to millions of Ohms. As the values can be so high, prefixes are used - 1000 Ohms = 1K, 1,000,000 Ohms = 1M. To reduce the chances of misreading a value, the decimal point may be replaced with a letter... for example, 1.2K may be written as "1k2" and 1.2 Ohms may be written as 1r2.

Dissipation.
In the process of resisting current flow, the resistor will create some heat. The amount of heat it will generate will increase with current or voltage.
The power which the resistor will dissipate as heat can be found from either the voltage across it or the current through it, and it's resistance value.
First square the voltage or current then:
   For current, multiply by the resistance.
   For voltage divide by the resistance.

Power rating.
Every resistor has a maximum amount of heat that it can dissipate without being damaged. This maximum is the power rating of the resistor, and is given in Watts.
Resistors for higher power applications are usually larger, and may state the power rating on the resistor.


Schematic symbol.
In circuit diagrams, resistors may be shown using one of the symbols on the right. The variable resistors, shown with an arrow, are usually user controls and available to the user. The preset types are usually inside the equipment, designed to be adjusted once and then left alone.