Runlevels in Linux


0 - halt
1 - Single user mode
2 - Multiuser, without networking
3 - Full multiuser mode
4 - unused
5 - X11 (Full multiuser mode with X-based GUI
6 - Reboot

And you can change the the current runlevel by editing the file
/etc/inittab

go to line number 18 showing as below line -

id:3:initdefault:

this means currently you are using the runlevel 3 for your linux operating system. If you want to change it to runlevel 5, then just replace 5 with 3 in above line and save the file & reboot once.
Then you will start with runlevel 5, which is a full GUI mode.