Run Levels and their meanings:

Value Explanation
0 Halt
1 Single-user text mode
2 Full multi-user with no networking
3 Full multi-user, console logins only
4 Not used (user-definable)
5 Full multi-user graphical mode (with an X-based login screen)
6 Reboot (Do NOT set initdefault to this)

CentOS 6:
List all services and their run levels: 

chkconfig --list

Starting service on boot

chkconfig httpd on
This turns on run levels 2, 3 and 4


Turning on specific run levels

Turning on levels 3,4 and 5 

chkconfig --level 345 httpd on

Turning off run levels

chkconfig --level 5 httpd off