Ethereum is a programmable blockchain. Rather than give users a set of pre-defined operations (e.g. bitcoin transactions), Ethereum allows users to create their own operations of any complexity they wish. In this way, it serves as a platform for many different types of decentralized blockchain applications, including but not limited to cryptocurrencies.
Like any blockchain, Ethereum also includes a peer-to-peer network protocol. The Ethereum blockchain database is maintained and updated by many nodes connected to the network. Each and every node of the network runs the EVM and executes the same instructions. For this reason, Ethereum is sometimes described evocatively as a “world computer”.
Whereas the Bitcoin blockchain was purely a list of transactions, Ethereum’s basic unit is the account. The Ethereum blockchain tracks the state of every account, and all state transitions on the chain are transfers of value and information between accounts. There are two types of accounts:
- Externally Owned Accounts (EOAs), which are controlled by private keys.
- Contract Accounts, which are controlled by their contract code and can only be “activated” by an EOA.
For most users, the basic difference between these is that human users control EOAs – because they can control the private keys which give control over an EOA. Contract accounts, on the other hand, are governed by their internal code. If they are “controlled” by a human user, it is because they are programmed to be controlled by an EOA with a certain address, which is in turn controlled by whoever holds the private keys that control that EOA. The popular term “smart contracts” refers to code in a Contract Account – programs that execute when a transaction is sent to that account. Users can create new contracts by deploying code to the blockchain.