Difference between Bar Diagram and Histogram:
(i) The spacing and the width or the area of bars are all arbitrary. It is the height and not the width or the area of the bar that really matters. But the width in a histogram is as important as its height.
(ii) Bar diagram can be drawn both for discrete and continous variables but histogram is drawn only for a continuous variables.
(iii) In bar diagram some space must be left between consecutive bars; but in histogram no space is, left between two rectangles.