Meaning of some router IOS command

User EXEC Mode
enable - Enter Privileged EXEC mode

Privileged EXEC Mode
copy running-config startup-config - Copy the active configuration to NVRAM.
copy startup-config running-config - Copy the configuration in NVRAM to RAM.
erase startup-configuration - Erase the configuration located in NVRAM.
ping ip_address - Ping to that address.
tracerouteip_address - Trace each hop to that address.
show interfaces - Display statistics for all interfaces on a device.
show clock - Show the time set in the router.
show version - Display currently loaded IOS version, hardware, and device information.
show arp - Display the ARP table of the device.
show startup-config - Display the saved configuration located in NVRAM.
show running-config - Display the contents of the currently running configuration file.
show ip interface - Display IP statistics for interface(s) on a router.
configure terminal - Enter terminal configuration mode.

Terminal Configuration Mode
hostname hostname - Assign a host name to device.
enable passwordpassword - Set an unencrypted enable password.
enable secret password - Set a strongly encrypted enable password.
service password-encryption - Encrypt display of all passwords except secret.
banner motd# message # - Sets a message-of-the-day banner.
line console 0 - Enter console line configuration mode.
line vty 0 4 - Enter virtual terminal (Telnet) line configuration mode.
interface Interface_name - Enter interface configuration mode.

Line Configuration Mode
login - Enable password checking at login.
password password - Set line password.

Interface Configuration Mode
ip addressip_address netmask - Set interface IP address and subnet mask.
description description - Set interface description.
clock rate value - Set clock rate for DCE device.
no shutdown - Set interface to up.
shutdown - Administratively set interface to down.