Want to learn more? I recommend working through: R for Data Science, R Cookbook, and R Graphics Cookbook.

Classes tell you what type a paticular variable is, whether numeric or logistic.

Use the class() function to find out a variable's class

# Create a variable called "crime", containing numeric data
crime <- c(1,2,4,5)
# find the class of the variable "crime"
[1] "numeric"