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's available under the MIT license for Windows, 32 and 64-bit GNU/Linux-based OSes and Mac OS X.
The client comes in two executables.
The bitcoin GUI provides an easy to use wxWidgets-based 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.