When a Linux system boots, it loads the kernel, all its drivers, and the networking servers, then the systemwill display a text login prompt. There, users can enter their user names and their passwords. But it doesn'thave to boot this way.There are 3 modes defined in most Linux distributions that can be used for booting. They are defined in/etc/inittab and have specific numbers. The first mode, also called runlevel 1, is single user mode. That modewill only boot the system for 1 user, with no networking. Runlevel 3 is the default mode. It will load the
networking servers and display a text login prompt. Runlevel 5 is the graphical mode. If you have X Windowinstalled and configured, you can use it to display a graphical login prompt.
The way to change this is to edit /etc/inittab and change the initdefault line:
id:3:initdefault:Changing a 3 to a 5 will make the system display a xdm graphical screen on bootup.

Linux boot up screen, Different runlevels,graphical login prompt