Accessing Linux System
To access and use your Linux system, you must carefully follow required startup and shutdown procedures. You do not simply turn off your computer. Linux does, however, implement journaling, which allows you to automatically recover your system after the computer suddenly loses power and shuts off. If you have installed the boot loader GRUB, when you turn on or reset your computer, the boot loader first decides what operating system to load and run. GRUB will display a menu of operating systems from which to choose.
If, instead, you wait a moment or press the ENTER key, the boot loader loads the default operating system. If a Windows system is listed, you can choose to start that instead. You can think of your Linux operating system as operating on two different levels, one running on top of the other. The first level is when you start your Linux system and where the system loads and runs. It has control of your computer and all its peripherals. You still are not able to interact with it, however. After Linux starts, it displays a login screen, waiting for a user to log in to the system and start using it. You cannot gain access to Linux unless you log in first.
You can think of logging in and using Linux as the next level. Now you can issue commands instructing Linux to perform tasks. You can use utilities and programs such as editors or compilers, or even games. Depending on a choice you made during installation, however, you may be interacting with the system using either a simple command line interface or the desktop directly. There are both command line login prompts and graphical login windows. Most distributions will use a graphical interface by default, presenting you with a graphical login window at which you enter your username and password. If you choose not to use the graphical interface, you are presented with a simple command line prompt to enter your username.