Access to a linux system usually requires a authentication. Most common method of authentication is the login process where a user enter a valid username and password. There are two types of login screens: Text based and Graphical. If you login by the Text based or command line interface you will be provided with a shell prompt whereas if you login from a graphical interface you will be provided with a desktop environment.
After successfully entering correct username and password combination you will get logged into the linux system. While entering password in text based login screen notice that password is not displayed.
Now after logging in you can run all run linux commands and in case you logged in by graphical screen you can run the terminal from applications > system tools > Terminal where you can run the linux commands.