Histogram
Histogram is a graphical representation of the Distribution of data
 Bins: the intervals used in a histogram. The data must be separated into mutually exclusive and exhaustive bins
 Cutpoints: the values that define the beginning and the end of the bins
 Frequency: the count of the number of the data values in each bin
 The peaks in the distribution are called modes
 so the variables you plot must be Quantitative Variables
Probability Density Function
hist(d$age, col="blue")
 Params

breaks=100
 how many bars in the histogram

 here we have 19 bins, and two modes
It can also be useful for Exploratory Data Analysis of two variables
Consider this example
 we have two classes of customers: $A$ and $B$
 and we want to build a model that can distinguish them
 so we can create a histogram that shows the distribution of age w.r.t. to class attribute

 can see that age and class are not independent: there is strong correlation between them:
 if age is lower then some value (say 30), all belong to class $A$
 if greater than other value  all always belong to class $B$
 can learn that just using a simple histogram
Cumulative Histogram
Usual histogram estimates the Probability Density Function
See Also
Sources