A probability density is a mathematical description of how likely a particular outcome is, when measuring something.
The normal probability function is one particular probability density commonly used as a reasonable approximate description of many circumstances in life.
A histogram is a chart which shows how frequently each result occurs in something. So, for example, if you measured the weights of 726 people, each to the nearest 5 kilogrammes, and drew a histogram, you might get something like Figure 47, with the most common weight in the middle. The height of each column shows how many people have that weight.
When you draw a histogram, the ...