Bitcoin is a peer-to-peer electronic cash system. It was described in a nine-page white paper published by Satoshi Nakamoto on 31 October 2008, and the network went live on 3 January 2009 when the first block — the "genesis block" — was created.
The problem it solves
Digital files are easy to copy. If money is just data, what stops someone from spending the same coin twice? Traditionally, a bank keeps the ledger and says no. Bitcoin removes the bank: thousands of independent nodes keep a copy of the same public ledger, the blockchain, and reach consensus on which transactions are valid.
Three ingredients
1. A shared ledger. Every transaction ever made is recorded in blocks linked together. Anyone can download and verify it.
2. Mining. New blocks are added through mining, which uses proof of work. This makes rewriting history extremely expensive.
3. Keys. Ownership is proven with a private key. If you can produce a valid digital signature, you can spend the coins. No name or account needed.
Fixed supply
There will never be more than 21 million bitcoin (because of rounding, the true maximum is just under 20,999,999.98). New coins enter circulation as the block reward, which is cut in half roughly every four years in an event called the halving. Each bitcoin is divisible into 100 million units called a satoshi (or "sat").
Why people care
Supporters value Bitcoin as censorship-resistant money that anyone can use with an internet connection, and as a scarce asset that no government or company can inflate. Critics point to its price volatility, energy use and limited transaction throughput. What is hard to dispute is that it was the first widely adopted working example of digital scarcity.
From here, the rabbit hole splits into four paths: the blockchain, mining, wallets and supply.