Linux is a Unix like operating system. It is open source and free. We might sometimes use the word "Unix" instead of Linux.
A user can interact with Linux either using a 'graphical interface' or using the 'command line interface'.
Learning to use the command line interface has a bigger learning curve than the graphical interface but the former can be used to automate very easily. Also, most of the server side work is generally done using the command line interface.