Runlevels define different system states and determine which services are running:

• 0: Halt/Shutdown - System shutdown
• 1: Single-user mode - Minimal system for maintenance
• 2: Multi-user mode without networking
• 3: Multi-user mode with networking (text mode)
• 4: Unused/User-defined
• 5: Multi-user mode with GUI
• 6: Reboot - System restart

# Check current runlevel
runlevel
who -r

# Change runlevel
init 3          # Switch to text mode
systemctl isolate multi-user.target  # SystemD equivalent

Modern systems use SystemD targets instead of traditional runlevels, but the concept remains similar.