Command line interface
Command Line Interface [kəˈmænd ˈlaɪn ˈɪntɚˌfeɪs]: A command-line interface (short CLI) is a way to interact with a program not by clicking with a mouse on icons, but by typing predefined commands.
Casual users prefer GUIs (graphical user interfaces) because you have to learn the commands for every program you want to use.
Advanced users often prefer CLIs because it supports a more powerful way to interact and automate user interaction.
There are many more programs who only have a CLIs because GUIs need more resources to develop and maintain. Before the 1970s using a computer terminal with commands was the primary mean of interaction.