A spark plug (also, very rarely nowadays, in British English: a sparking plug) is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol gasoline, Ethanol, and Liquefied petroleum gas by means of an electric spark.
That's from WIKIPEDIA.
In simple you should clean and change your spark plugs at regular intervals, which should be mentioned in your car manual.
Spark plugs range from 100rs (1 plug) for copper one, to 700-800rs for 4 tip iridium ones.
platinum and iridium one are tough metals last long and have better conductivity, it means you will be giving a proper spark to your car, especially beneficial for CNG cars.
The best spark plugs (Iridium one) can be used for up to 120,000 miles or more then 190,000 kms but they should also be checked and cleaned at proper intervals.
Iridium is 8 times tougher then platinum with high melting point and less resistance.
You should definitely not USE a 100rs spark plug for more then 10,000 kms.
And remember a IRIDIUM spark plug is good, if you can afford one, but it would not convert your alto or civic into type r or 911.