The whitepaper, "Bitcoin: A Peer-to-Peer Electronic Cash System" authored by Satochi Nakamoto started it all. And if a normal sucess has many fathers, then super-sucess begets many heirs claiming the throne.
BTC has for a long time been the leading direct descendant of the original Bitcoin software client, but there are forks with different features that claim the throne, most recently and notably Bitcoin Cash BCH. Each camp has strong opinions so it's important to understand the differences.
Bitcoin.org was originally owned by Satoshi and then passed to the community. There are now other organizations that claim Bitcoin association, but non have a fun video with someone paying 0.01 BTC for a coffee. Depending when you're reading this, that's an expensive coffee (ignoring any fees arguments and talk of segwit).