There are two variations of the official bitcoin program available; one with a graphical user interface (usually referred to as just “Bitcoin”), and a 'headless' version (called bitcoind). They are completely compatible with each other, and take the same command-line arguments, read the same configuration file, and read and write the same data files. You can run one copy of either Bitcoin or bitcoind on your system at a time (if you accidently try to launch another, the copy will let you know that Bitcoin or bitcoind is already running and will exit).
Give Bitcoin (or bitcoind) the -? or –help argument and it will print out a list of the most commonly used command-line arguments and then exit:
bitcoin [options] bitcoin [options] <command> [params] Send command to -server or bitcoind bitcoin [options] help List commands bitcoin [options] help <command> Get help for a command
-conf=<file> Specify configuration file (default: bitcoin.conf) -gen Generate coins -gen=0 Don't generate coins -min Start minimized -datadir=<dir> Specify data directory -proxy=<ip:port> Connect through socks4 proxy -addnode=<ip> Add a node to connect to -connect=<ip> Connect only to the specified node -server Accept command line and JSON-RPC commands -daemon Run in the background as a daemon and accept commands -? This help message