You may use the following commands to configure a description on a Cisco router or switch. Please note that this will not affect the performance of the router or switch but I recommend doing this for easier administration.

===BEFORE===

router-1#show running-config interface FastEthernet0
Building configuration…

Current configuration : 68 bytes
!
interface FastEthernet0
no ip address
shutdown
speed auto
end

===CONFIGURING THE DESCRIPTION===

router-1#config t
Enter configuration commands, one per line. End with CNTL/Z.
router-1(config)#interface FastEthernet0
router-1(config-if)#description Local Area Network (LAN) Link – 1
router-1(config-if)#^Z

===AFTER===

router-1#show running-config interface FastEthernet0
Building configuration…

Current configuration : 115 bytes
!
interface FastEthernet0
description Local Area Network (LAN) Link – 1
no ip address
shutdown
speed auto
end

===SAVE THE CONFIGURATION===

router-1#copy running-config startup-config
Destination filename [startup-config]?
Building configuration…
[OK]

I understand that this is a basic task so please stay tuned for more advanced configurations in the future.