what is the difference between distance travelled by a body and its displacement, explain with the help of a diagram

Distance is the actual path covered by a body.                   Displacement is the shortest distance covered by a                                                                                             body. It can be negative and is a vector quantity.
It can never be negative and is a scalar quantity.
 

If you don't understand by the above answer then- 
Imagine there is a rectangular park(ABCD) and there is a track along it's perimeter which is 220 m. Now, firstly you travel on that track and covers a distance of 110 m (from A to C) and then again you travel from point A to C via the diagonal joining these to opposite corners and this diagonal is the shortest distance (displacement) from point A to C which is the square root of 6100 = 78.102 approx.