Ether is the currency of the Ethereum platform. Anyone who wants to use Ethereum platform will have to use Ether. Ether can be traded around the web and mined exactly in the same way as Bitcoin is done.
Ethers supply and rate of issuance has been decided by the statistics gathered from the currencies 2014 presale.
There is this misconception that Ether is a currency, but its founders say that Ether is not a currency as much as it is crypto-fuel, which means that its a token that has one main use to pay for the Ethereum platform. This means that you probably wont be able to buy stuff with Ether online. However you can still trade it and invest in it in hopes its price goes up.