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:
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List all services and their run levels:
<sxh bash>
chkconfig --list
</sxh>
=== Starting service on boot ===
<sxh bash>
chkconfig httpd on
</sxh>
This turns on run levels 2, 3 and 4
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=== Turning on specific run levels ===
Turning on levels 3,4 and 5
<sxh bash>
chkconfig --level 345 httpd on
</sxh>
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===Turning off run levels ===
<sxh bash>
chkconfig --level 5 httpd off
</sxh>