Transformers
Transformers have two or more windings (For more information, read about coils) wrapped usually around a common core made form laminated iron sheets.
The primary coil is the input side of the transformer and the output side is callled the secondary coil. If the current flowing through the primary winding is fluctuating, a current will be induced into the secondary winding. Note, a steady current (DC) will not be transferred.
How Transformers Work
Transformers can transform voltage and current to a lower or higher level. If the voltage of a signal is increased, the current is reduced, and if a tranformer decreases the voltage, it increases the current. This highlight an important rule, the power flowing out of a transformer, will not exceed the power going into the transformer.
Turns Ratio
The 'Turns Ratio' of a transformer is the ratio of primary to secondary turns. This determines the transformer's voltage ratio. There are three possible types of ratios in transformers, 1:1 ratio (where voltage and current are not changed - known as an isolation transformer), Step-up, where there are more turns on the secondary winding, so a ratio of 1:5 will increase 10 volts in the primary into 50 volts at the secondary, and a Step-down, where the voltage is decreased.