What is the difference between the height specifications "PDF" and "Probability" for Histogram built-in function?

maybe this question is something conceptual but I need to clarify it.

Given a data I can use Histogram to plot the distribution of the frequencies of the data. If I need the relative frequencies I can use the height specification "Probability" for the Histogram built-in function. In principle we don’t know the probability distribution function of the population, but our histogram approximates that distribution. I really need to understand what represent the height specification "PDF" for the histogram and how are calculated those heights.

For example, here are the two different height specifications for the some data

datos={19, 10, 10, 8, 8, 8, 7, 6, 6, 6, 5, 5, 5, 5, 5, 5, 5, 5, 5, 3, 3, 2, 
2, 1, 1, 1, 1};

Thanks in advance!