The battery is the sole source of electrical power for the car. The battery is charged during vehicle operation by the alternator. If the battery is not charged sufficiently, you will not be able to start the car or use any electrical components (lights, radio, etc.)
Symptoms of a dead battery:
Frequency of replacement: If operating under normal conditions a good battery should easily last for 2 - 2.5 years.
Corrosion to battery cable is one issue that you should be careful about. A white or blue color deposit you see on the terminals is nothing but dried acid which will corrode the battery cable and terminals. It is advisable to use Vaseline or dielectric grease to reduce corrosive effect.