While people usually say Ethereum, it is not Ethereum that you buy. You buy Ethereum Network Tokens, called Ethers.
The tokens are necessary to process smart contracts. Each smart contract (transaction is also a smart contract, albeit not that smart) consumes a little bit of Ether.
How much Ether gets consumed depends on complexity of smart contract and how much the Ethereum node network is stressed.
Ether Classic (ETC) is a cryptocurrency that was created due to a disagreement with the way original Ethereum network conducted business.
The catalyst for the split was due to a hacker finding a way to steal $60m worth of Ether coins. The supporters of ETH said that the code should be altered and the stolen coins should be refunded, while the supporters of ETC said that “code is law” and whatever happened happened – even if it was not ethical.
So in real life what happened is that ETC was the same original Ethereum Network code and ETH forked their code to create a new Ethereum Network.
Therefore two identical competing currencies were created that had two different exchange rates. Today ETH’s priced around 10 times higher and is more common around the general cryptocurrency community, though the basic concept is the same.