Ether is the currency of Ethereum. Every activity on Ethereum that modifies its state costs Ether as a fee, and miners who are successful in generating and writing a block in a chain are also rewarded Ether. Ether can easily be converted to dollars or other traditional currencies through cryptoexchanges.

Ethereum has a metric system of denominations used as units of Ether. The smallest denomination or base unit of Ether is called wei. The following is a list of the named denominations and their value in wei which is available at https://github.com/ethereum/web3.js/blob/0.15.0/lib/utils/utils.js#L40:

varunitMap={
   'wei' : '1'
   'kwei': '1000',
   'ada': '1000',
   'femtoether': '1000',
   'mwei': '1000000',
   'babbage': '1000000',
   'picoether': '1000000',
   'gwei': '1000000000',
   'shannon': '1000000000',
   'nanoether': '1000000000',
   'nano': '1000000000',
   'szabo': '1000000000000',
   'microether': '1000000000000',
   'micro': '1000000000000',
   'finney': '1000000000000000',
   'milliether': '1000000000000000',
   'milli': '1000000000000000',
   'ether': '1000000000000000000',
   'kether': '1000000000000000000000',
   'grand': '1000000000000000000000',
   'einstein': '1000000000000000000000',
   'mether': '1000000000000000000000000',
   'gether': '1000000000000000000000000000',
   'tether': '1000000000000000000000000000000'
};