"Liber Abaci"

Upon his return to Italy, Fibonacci wrote Liber Abaci in 1202. It was in this work that he introduced the Latin World (Europe) to the Hindu-Arabic number system (0, 1, 2,..., 9). Previously they had used the Roman Numeral numbering system. Liber Abaci  was written on a practical level that included important examples and strategies for merchant and bankers to help them do their mathematical calculations in a more efficient and easier way. Although Fibonacci is more so remembered for the "Fibonacci Sequence" (discussed below), the work he did in spreading the decimal numbers can be argued as his greatest and most influential achievement in regards to the impact it had on future mathematics.

Fibonacci Sequence: 1 1 2 3 5 8 13 21 34 55 89 144 233....
The next number in each sequence is the sum of the previous two numbers before it.
Example: 1+0=1, 1+1=2, 2+3=5, 5+3=8, 8+5=13, etc.

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Sources
Gies, Frances Carney. "Leonardo Pisano (Italian Mathematician)." Encyclopedia Britannica Online.  Encyclopedia Britannica, n.d. Web. 27 Feb. 2014.

Knott, R. "The Life and Numbers of Fibonacci." Plus.maths.org. N.p., n.d. Web. 27 Feb. 2014.