The command-line interface is a non-graphical user interface. It usually consists of a prompt and is controled by entering in characters and hitting "enter" or "return" Most UNIX-type tools are operated this way and most remote administration is command-line.
Depending on your operating system, it can look a number of ways:
Windows DOS Prompt, COMMAND, or CMD function:
Different UNIX shells have different prompts, either a $, #, or %.