Command Line Interface
What all computers were before GUIs came about. CLI is an interface that only uses text. Some stupid people and groups (like micro$oft, apple and linspire) try to get rid of CLI taking a lot of power away from users. CLI is still used a lot on UNIX and UNIX-like OSs.
If you want to see what's so great about CLI, use GNU Bash on a GNU/Linux system.
GNU Bash is a great CLI.