The capital Latin letter R is used in mathematics to represent the set of real numbers. Usually, the letter is presented with a "double-struck" typeface when it is used to represent the set of real numbers.
The set of real numbers contains the set of rational numbers as well as irrational numbers like pi, e, and the square root of two.
The Latin Alphabet is a collection of 26 symbols that form the basis of the English language. The alphabet's symbols are used throughout mathematics to represent variables, constants, and coeffecients.
The latin letter r is used in math as a variable. It appears in geometric equations as a variable to represent the radius of a circle.