Bitcoin Core is an open source project which maintains and releases Bitcoin client software called “Bitcoin Core”.
It is a direct descendant of the original Bitcoin software client released by Satoshi Nakamoto after he published the famous Bitcoin whitepaper.
Bitcoin Core consists of both “full-node” software for fully validating the blockchain as well as a bitcoin wallet. The project also currently maintains related software such as the cryptography library libsecp256k1 and others located at GitHub.
Anyone can contribute to Bitcoin Core.