Over two thousand years ago, the ancient Romans adapted the Etruscan way of writing numbers by using letters of their own alphabet (the Etruscans had in turn based their system on that used by the Greeks). The resulting numerals are very cumbersome to work with, and today the numbers in common use are modeled on those developed by the Arabs over a thousand years ago from numerals developed in India. You don’t see Roman numerals much any more except in outline numbering, although until about twenty or thirty years ago, the year of copyright in a book was sometimes in Roman numerals.
There was a lot of variation in the use of Roman numerals by the Romans, but beginning in the Middle Ages, the numerals, like the Latin language, became formalized, giving us the system used today.
Below is a chart showing some conversions between Arabic and Roman numerals. To make a Roman numeral not on the chart, just string together the parts: 475 would be 400 (CD) followed by 70 (LXX) followed by 5 (V), CDLXXV.
Here are modern European, Arabic, and Persian numerals; all three developed from an older style that the Arabs adopted from India (and to which they added zero). On some computer systems you may see empty boxes instead of the Arabic and Persian examples.
Citation: Clark, Curtis. 2001. BIO 190 - Understanding Roman Numerals. California State Polytechnic University, Pomona, http://www.csupomona.edu/~jcclark/classes/bio190/mm.html.