It's well known that the Romans used Latin letters for writing numbers.
It is considered that the Roman numeral system is a classic example nonpositional numeral systems, i.e. the numeral systems in which the value of a figure is independent of it's position.
We remind you that in Roman numeral system I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1000.
For example, number 3, is written as III in Roman numerals.
Although, everything is not so simple and it's a nonpositional numeral system because there is an additional rule that modify the value of a digit according to it's place.
That rule forbids the use of the same digit 3 times in a row. That's why 3 is III but 4 is IV and I(1) placed before the larger digit V(5) means subtraction so it's actually -1.
Anticipating the obvious question, we'll say that the largest number allowed in this numeral system is 3999 because there is no 5000 and 10000 and to write 4000 you have to use MMM which is forbidden. How did they count further - still a mystery.
Here is two calculators below - for numbers ranging from 1 to 3999 to roman conversion and vise versa.