Distance (d) is what is known as a scalar quantity i.e. it has magnitude but does not represent any information about direction. There are some important differences between distance and displacement which are illustrated below.
The magnitude of the displacement can be different to the distance travelled!
Displacement is the straight line distance of an object in a particular direction. However objects do not always move in straight lines and therefore the distance travel will often be greater. This is a common situation when travelling by road where the distance travelled is generally greater than the final displacement (unless you travelled along a perfectly straight road to get there!). (Only when an object moves in a constant direction along a straight line will the distance travelled be equal to the displacement.)
The final displacement can be zero even when an object has travelled a long distance!
When you make a return journey (i.e. home to work/school and then back again at the end of the day) the difference between distance travelled and displacement is very evident. Your final displacement will be zero (because you have returned to the point that you started off from), however the total distance you have travelled over the journey might be very large!
Distance gives no information about direction!
Displacement is a measure of straight line distance in a particular direction from a specified reference point. e.g. to say that a ship is located 25km due west of Lighthouse is stating its displacement. If we were only told that it was 25km from the Lighthouse then even if we assume that this is the straight line distance (which generally does not have to be the case). we would still only know that it was somewhere along a perimeter of 25km radius centred on the Lighthouse!