Important question, what is the meaning of the term (sub interface) and why to use it?

A subinterface is a logical interface which is created by dividing one physical interface into multiple interfaces. However, the Parent  Physical Interface  is used for data transmission. Sub-Interface can be used for variety of Purpose, Inter-Vlan Routing, Frame Relay and ATM. For Large number of VLANs, Layer3 Switch is preferred in comparison to Router on a stick configuration due to Switch Backplane Bandwidth.

Subinterfaces are created by splitting up a physical interface into a number of logical interfaces so that we can configure different IP's or properties(point-to-point connection, multipoint connection, VLAN, etc) for each sub interface

What:  A sub-interface is a logical interface that uses the "parent" physical interface for actually moving the data.

Why: If we had a router with only1 physical interface, but needed to have the router connected to two IP networks, so that it could do routing, we could create2 logical sub interfaces, assign each sub interface an IP address within each subnet, and we can then route between them.

py   switchport trank encas

I will go with this video: http://www.youtube.com/watch?v=RXOiS6IfZeo

Many Thanks. Muhammad Tahir 

use sub-interface wen you need to split one physical interface to many logic interface

it is important espishally in routing between VLANs.thanks for question

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My answer might be simillar to others because they have already explained very nicely but just want to add0.02 cents.

subinterface is a logical interface which is used to divide one physical interface into multiple interfaces. It uses the parent physical interface for sending and receiving data.

If we use one router to route the traffice between different ip networks, other than installing different cards you can create sub interfaces and assign each sub interface an IP address within each subnet and then route the data between those subnets

A Subinterface is just like the gates on the Airport. There is one Airport but so many planes distined to different places around the world from different gates. Gates are subinterfaces and Airport is one physical interface.

I hope this real world example might help to clear the idea.