The Roman numerals are: I, V, X, L, C, D, M - in sequence, these stand for units, five, ten, fifty, hundred, five hundred and thousand. Hence, 3, say, is III...25 is XXV...57 is LVII and 2003 is MMIII. The only somewhat unusual element is where items appear to be in reverse order. 9, for example is IX - ie one before ten and 40 is XL - ie ten before fifty.
Hi Liz - QM forgot to point out how writing the letters backwards and/or drawing a line over the top of them affects their value. I'm consulting Chamber's 20th Century Dictionary... and if you're watching kids, guess what I'd like for Father's Day.