Since Ethereum is built on top of Bitcoin, it is considered to be a Bitcoin altcoin. Ether to Ethereum is similar to BTC to Bitcoin. Ethereum is the name used when referring to the protocols, the blockchain, the client software, and the mainnet.

The Ethereum mainnet is the blockchain network that is used by clients for transferring a digital asset from a sender to a recipient. In other words, it is the network where actual transactions take place on a distributed ledger. Mainnet is equivalent to a production environment. The Ethereum testnet is for development. As mentioned at https://www.ethernodes.org/network/2, as of October 8, 2018, the mainnet had 13,662 nodes and the testnet had 29 nodes. Since the actual transactions take place on the mainnet, Ether has a real value only on the Ethereum mainnet. In other words, on the testnet, it is worth nothing. Ether coins (ETH and ETC) are listed and exchanged in tens of thousands of digital currency markets. Their prices vary greatly. For example, on October 8, 2018, ETH was traded at around $223 and ETC at $11.

Ether can be transferred between addresses (accounts). It is used to pay miners for their computational work; they are paid in transaction fees and also for gas consumption resulting from executing a transaction. Here, the concept of gas is essential for Ethereum and it is discussed later in more detail.

Ether is the largest denomination. There are other units. The smallest is called WEI, named after the digital money pioneer, Wei Dai, who is the inventor of B-money. B-money was his proposal for an anonymous, distributed electronic cash system. Other units include Gwei, microether, and milliether. They all have a second name. For example, milliether is also called finney, named after another digital money pioneer, Harold Thomas Finney II, who in 2004 wrote the world's first implemented cryptocurrency, RPOW (reusable proofs of work) before Bitcoin. The following table gives the conversion rate between ether and other units: