There are many forms of lighting available for aquariums, from simple fluorescent tubes to high-powered metal halide bulbs.
There are a number of functions of lighting, from simply being able to see your fish, to providing an energy source for plants, and in marine aquariums, for corals.
Not all light is the same and different lights emit differing light spectrums, or colours. The most common form of lighting, fluorescent tubes, are available in a number of different styles, each emitting a different light spectrum. The varieties of fluorescent tubes are designed for different purposes such as for enhanced plant growth, mimicking natural sunlight, or reflecting the colours of fishes.
Lighting is also important for providing a daily light-dark cycle, which the fish depend upon for normal function and behaviour. The fish in your aquarium will adapt to a regular lights on/off time, adapting their behaviour to this regular cycle by learning when it is time to 'settle down' for the night, or when it is nearly feeding time.
When some fish breed, and care for the young, they often 'herd' the young fry to a safe place when it is nearing lights-off time, to do this they must have adapted to a regular light cycle.
To provide such a regular pattern of lighting, it is a good idea to plug your lights into an inexpensive timer unit, which will automatically switch your lights on and off. In most cases, lighting should be provided between 10-14 hours a day.