Ether is a token that runs on the Ethereum network, which is a way that anyone can pay for transactions on the Ethereum network and support it.
Ether is often called "cryptocurrency", but it does not mean the Ether is a currency like Bitcoin.
Its primary function is to provide "fuel" for the computer resources needed to run applications distributed on the Ethereum network. However, the Ether can be purchased and sold just like the oil. Its price fluctuates based on supply and demand.