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That is the entire alphabet. Every Roman numeral ever written is some arrangement of those seven letters, read left to right, largest first.
Subtractive notation. When a smaller symbol sits in front of a larger one, you subtract instead of adding — so IV is 5 minus 1, and IX is 10 minus 1. It keeps numbers short. The rule is narrower than people assume: only I, X and C can be subtracted, and only from the next two values up. So 99 is XCIX, not IC. Clock faces often still use IIII for four, purely for visual balance against the VIII opposite it.
There is no zero and no way to write a fraction or a negative number. Standard notation also stops at 3,999 (MMMCMXCIX), because there is no symbol above M — larger numbers needed a bar over the letter to multiply it by a thousand. Try doing long division in it once and you will understand why Europe switched.
Convert either direction here — type a number to get numerals, or numerals to get the number.