There are two versions of bitcoin official client:
One version has graphical interface,it is usually called Bitcoin-Qt,the initial must be capital.
Another brief version is called bitcoind.
They are compatible to each other,which has the same arguments of command line,the same config files and the same data files.
You can only run one of Bitcoin-Qt and bitcoind in a computer. If you accidentally run both of them,it will remind that another client is already running.