They're aptly named as well; spark plugs are simply insulated plugs that are screwed into an internal combustion engine's cylinder head to deliver the spark that ignites the mixture of air and fuel in the combustion chamber. Spark plugs also transfer heat away from the combustion chamber.
Spark plugs are incredibly inexpensive, often costing less than ten dollars apiece. Now you may need to replace several at once, but it still won't cost very much. The typical amount you will pay for spark plugs is between $16-$100, while for labor on a spark plug replacement you can expect to pay around $40-$150.
If you check your owner's manual, you'll probably find that your automaker recommends you replace your spark plugs roughly every 30,000 miles. That's fine if you're using stock spark plugs. However, the actual timing of replacement will vary depending on other factors.
There are two types of spark plugs: conventional and long-life. Conventional spark plugs need to be replaced every 30,000-50,000 miles. Long-life (iridium- or platinum-tipped) spark plugs need to be changed between 60,000 and 150,000 miles, depending on the vehicle.
Without spark plugs, your car won't start. The health of your spark plugs is directly related to the health of your car's engine. It's essential that you keep the spark plugs in good health and replace them as needed so that your car can continue to run at maximum health.