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 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.
The set of real numbers symbol is a Latin capital R presented in double-struck typeface.
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.