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Distance is a scalar quantity. It is the total length of path travelled, even if that path is constantly changing direction. It is measured in metres in the S.I. unit set and is often given the symbol 'd' in equations.

Displacement is a vector quantity. It is the length and direction of a straight line drawn from the starting point to the finishing point of a journey. It requires both a unitof distance and direction (often a 3-figure bearing from North) to be stated, unless the displacement is zero. It is given the symbol 's' in equations.

To understand the difference let us consider the toy train track on the right.

If the train travels from A all of the way around the track, it will have travelled 1.0 m of distance, but as it finishes where it started, its displacement will be zero!

But, if it travels from A to B, it will have travelled a distance of 50 cm but its displacement will be the diameter of the track 1.0/π = 32 cm.