The Original Bitcoin client is the first bitcoin client in the network's history. It is also the official client of the bitcoin project.
It comes in two executables.
The bitcoin GUI provides an easy to use interface, allowing to receive and send bitcoins, and generate bitcoins using the computer's CPU power.
The bitcoind CLI interface runs as a daemon, and provides a JSON-RPC interface allowing to control it locally or remotely. Various commands are made available by the API.