Not the first cryptocurrency, but definitely the first to go mainstream. It still holds the highest market cap, and is widely accepted as the most reliable and robust cryptocurrency in existence. It was released in 2009.
Wikipedia offers a great explanation of the protocol, which is similar in a number of cryptocurrencies.
"The system is peer-to-peer, and transactions take place between users directly, without an intermediary. These transactions are verified by network nodes and recorded in a public distributed ledger called a blockchain. Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency. Besides being created as a reward for mining, bitcoin can be exchanged for other currencies,products, and services in legal or black markets."