What is the difference between a bar graph and a histogram?

There are two differences, one is in the type of data that is presented and the other in the way they are drawn.  In bar graphs are usually used to display "categorical data", that is data that fits into categories.
 Histograms on the other hand are usually used to present "continuous data", that is data that represents measured quantity where, at least in theory, the numbers can take on any value in a certain range.   The difference in the way that bar graphs and histograms are drawn is that the bars in bar graphs are usually separated where in histograms the bars are adjacent to each other. This is not always true however. Sometimes you see bar graphs with no spaces between the bars but histograms are never drawn with spaces between the bars.