Why are turns banked? I have seen everywhere that where on road, on railway track when there is a  turn  then  one side of that road or railway track is below the surface or some what down then actual surface.   
Any one tell me why this type of mechanism is used on turn ?
 A: In high school physics classes this is often explained as follows: 
 
When the road is not inclined (leftmost figure), the force of gravity and centrifugal force due to the curve add up such that the occupant(s) of the vehicle is pushed to the outside. When the road is inclined (rightmost figure), those same forces push the occupants "into their chairs". The inclination is there mostly to increase comfort (same holds for cargo, of course).
A second thing to consider is slipping. The same analysis shows that the vehicle also experiences a force slightly to one side (leftmost figure), which may make it more vulnerable to slipping when compared to the inclined road (rightmost figure). This is more important when typical vehicle types and their speeds are expected to be such that this will be a problem--on certain race tracks, for example. 
In the case of public roads and highways etc., inclined roads can indeed assist cars to make a turn, improving the safety of that turn. However, public roads being public, not only cars drive on it. For example trucks also use that same road. They generally drive slower than cars and thus have different demands on the degree of road inclination. And in case of a traffic jam, all cars and trucks could slip to the side :) For these and many other reasons, public roads are not usually inclined. 
In the case of railroads, inclination is used for the same reasons. Here, wear and tear are more important considerations--a train usually is quite heavy, and the centripetal force exerted by a non-inclined rail onto the train makes one side of the rail wear off much more quickly than the other side. Inclining the ail will balance the wear on both sides, increasing the time between replacements of the rail and reducing the risk of failure. 
In reality, there may be a wide variety of reasons other than the ones I mentioned why roads/rails are inclined. On soft beddings for example, the road may simply have sunken more on one side due to the higher average load. Or there may be known water issues in case of heavy rainfall, so that inclination will help remove the water to the "desirable" side. Or, the terrain was already inclined and it was simply cheaper to leave that in. etc. 
A: This tilt is done to compensate centrifugal force to some extent thus allowing higher speeds at tight curves. Without it train would tilt somewhat in direction away from arc centre. This is in some range acceptable but if it's too much it will derail. If you introduce tilt into railway itself you decrease amount of train tilting. 
As a side note - it seems more of an engineering question than pure physics.
A: I am somewhat not satisfied with the explanation of accepted answer. I found no mention of normal force which is key to understanding this situation.
Also the outward force is centrifugal force, as the frame is rotating.
I'll make my point real quick.
No banking
In this case, normal reaction is orthogonal to centrifugal force, so it has no effect on it. Whatever slipping is prevented, is prevented solely by friction 

Friction alone provides the required centripetal force

With Banking
In this case, a component of normal reaction acts inwards, opposite to centrifugal force. This helps in reducing wearing of tyres, as friction would reduce.

Friction + Component of normal force provide the required centripetal force

Also it would help prevent toppling, as torque of the body about line joining the outerwheels reduces
Take a look at this:

