Bitcoin Core is the name of an open-source software tool that enables the use of this Bitcoin currency. Bitcoin Core is programmed to decide which block chain contains valid transactions. The users of Bitcoin Core only accept transactions for that block chain, making it the Bitcoin block chain that everyone else wants to use. It is these users who keep Bitcoin decentralized. They individually run their own Bitcoin Core full nodes, and each of those full nodes separately follows the exact same rules to decide which block chain is valid.