There r differences in the runlevels according to the operating system. Nearly 7 runlevels r supported in the standard Linux kernel (i.e., core of the operating system). They are:
0 - System halt; no activity, the system can be safely powered down.
1 - Single user; rarely used.
2 - Multiple users, no NFS (network filesystem); also used rarely.
3 - Multiple users, command line (i.e., all-text mode) interface & Network support; the standard runlevel for most Linux-based server hardware.
4 - Not used/User-definable; For special purposes.
5 - Multiple users, GUI (graphical user interface); the standard runlevel for most Linux-based desktop systems.
6 - Reboot; used when restarting the system.

As u mentioned here 3,4 and 5, for ur info 4 is not used but yes, 3 and 5 are generally used for networking purpose.