Bitcoin is the first peer-to-peer decentralized cryptocurrency. It is operated by the network without any central authority like a government or a bank. Transactions are processed by the participants in the network called miners who get rewarded for their work in the form of Bitcoin. This is the only way new coins can be produced.
The technology that makes all of this possible is called Blockchain, an immutable append-only data structure used for keeping all transaction data transparent and secure. It is secured by SHA-256 Proof Of Work (POW) algorithm with a limit of 21 million mineable coins. The average block time is 10 minutes.
Bitcoin is the cryptocurrency with the biggest market cap and is the most widespread accross the world. It can be traded to most of the major fiat currencies and is the entrypoint to the majority of the smaller coins as well.