Cisco IOS alias can be used to shorten commonly used commands to a command alias.

To save time and the repetition of entering the same command multiple times, you can use a command alias. An alias can be configured to do anything that can be done at the command line, but an alias cannot move between modes, type in passwords, or perform any interactive functions
Cisco DOC

Default Command Aliases

Cisco IOS includes some built-in command aliases. Depending on different IOS version, default alias may be slightly different
You can check the built-in alias using the following command in privileged mode

Router#show aliases
Exec mode aliases:
  h                     help
  lo                    logout
  p                     ping
  r                     resume
  s                     show
  u                     undebug
  un                    undebug
  w                     where

Router#

As you can see there are couple of aliases already defined.

Configuring Alias

Suppose you want to configure an alias named int_br which lists all the interface and brief details about it.
Under config mode, use this command

Router(config)#alias exec int_br show ip interface brief

Now we can use this alias in privileged mode

Router#int_br
Interface                  IP-Address      OK? Method Status                Protocol
FastEthernet0/0            unassigned      YES unset  administratively down down
FastEthernet0/1            unassigned      YES unset  administratively down down
Router#

int_br = show ip interface brief

Here are some more useful aliases

Router(config)#alias exec c config terminal
Router(config)#alias exec w write memory
Router(config)#alias exec sr show ip route

Summary

Alias is very useful and time saving IOS command. Long repetitive commands can be shortened with alias. This can be very useful in CCIE labs or where time is limited.
Check out Cisco DOC for further details.