The number after Crystal refers to the Lovibond rating of the malt. This rating can be used in conjunction with formulas to estimate the color of the finish beer. Beersmith has an excellent blog write up about Beer Color.
Now since maltsters have to kiln or dry roast the malt to get it that color rating, the flavor profile of the crystal also changes. For example a Crystal 10 will give the beer a more sweet, slightly caramel flavor, whereas Crystal 60 will give you a more pronounced, fuller caramel flavor. When you start getting into the higher crystals like 120, you start to get more toffee and raisin.