What is a Blockchain?

Before diving into what Ethereum is we should first understand what a Blockchain is.

According to Wikipedia:

A blockchain is a growing list of records, called blocks, that are securely linked together using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkle tree, where data nodes are represented by leafs).

For a developer, however, I thought this would be a better definition:

A blockchain is a cryptographically secure single instance open database with share-state

I'll break down each of these in part:

"Cryptographically secure" means that each entry into the blockchain is secured using mathematical algorithms that are incredibly hard to hack. This means that to "change" what has already been entered into the system (it's history) would be very very difficult.

Cryptographic here means that it uses Cryptography to ensure no fake transactions occur, one cannot erase transactions, and the history of the state is not changeable.

"Single instance open database" means that there is a single canonical instance of the database that tracks all the transactions being created in the system. This is the single source of truth that everyone can agree on.

"Shared state" and "open" here means that this single source of truth is accessible by anyone and everyone.

You can think of it like an open database that everyone can read from, write to, and no one can hack.

The blockchain database stores "entries" or "transactions" that is updated and shared across many computers in a network. Every time a new set of transactions is added, its called a “block” - hence the name blockchain.

Most blockchains are public, and you can only add data, not remove.