A vehicle's alternator is the source of energy for the battery, ignition system, lights and all other electrical components. They are essentially generators driven by the engine via a belt that provide the electricity necessary to run a vehicle.
When an alternator goes bad, or is putting out less than desirable voltage, the car's battery will often end up running dry, requiring jumps. Sometimes this can cause the battery itself to fail. In worst case scenarios the alternator will put out so little energy that spark plugs cannot fire and the engine stops altogether.
Replacing an alternator varies in difficulty, with some vehicles requiring 15 - 20 minutes of labor and others taking hours. It is certainly a job accomplished easily by novice mechanics on the simpler setups. That being said, always consult with an automotive professional before performing any work if you are unsure of anything!
We have a large selection of new and rebuilt alternators: