The Romans used only seven letters to express all their numbers. The combination of a letter and its position could represent any number.
If you think that is an overambitious feat, you are correct.
They also use a line above the letter, so the numbering system actually represents our own very closely with fourteen different symbols.
The major difference is how they treat notion of zero.
Conversions:
I=one
V=five
X=ten
L=fifty
C=one hundred
D=five hundred
M=one thousand