I see that almost all of Honda's cars have an "i-VTEC" logo at the back. I googled it and came to know that it has something to do with different cam profiles.
Can someone explain? A diagram or animation would be helpful.
And why does Honda do it?
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In addition to Zaid's answer.
VTEC - Variable Valve Timing and Lift Electronic Control)
In essence, it selects between two different cam profiles depending on the RPM.
Here's an animation -
This is how a cam controls poppet valves. Read about cam profile here
For volumetric efficiency we want the intake stroke to fill the cylinder with the max - air-fuel.
I'l jump to the need for having a different cam profile at higher RPM's
Intake - Valve isn't open long enough for the air (or air-fuel mix) to completely fill up the cylinder under the given pressure gradient. We could help this situation by using a different cam profile which allows the cam to be open for longer. This means the intake valve is open even after the piston reaches the bottom of the intake stroke. Air at this velocity has sufficient momentum to continue to push and compress into the cylinder even as the piston is moving up for the compression stroke.
Exhaust - we could do something clever here as well. If we open the intake stroke as well (overlap) at the end of the exhaust stroke we end up using the momentum of the exhaust to suck in air from the intake port!
Lift control basically equates to changing the effective area of the 'hole'. At higher RPM's we would benefit by using a bigger 'hole' and hence the cam profile is such that it also pushes the poppets deeper. Since the size of the port is a tuned parameter, using a bigger hole all the time will result in the engine running rough.
So vtec's switch between two distinct cam profiles which is selected by the ecu depending on the RPM. There is a distinct surge that can be felt at the switch-over point.
But of course we could benefit even further by having a continuous control over lift and timing.
This diagram does a decent job of explaining VTEC:
In a nutshell (paraphrasing the Wikipedia article), you have two cam profiles, one designed for low-RPM operation and fuel efficiency, the second (taller cam) for performance. The ECU decides when to select which profile based on several engine parameters by actuating the locking pin that runs through the rocker arms.