What are the differences between candy thermometers and meat thermometers?
I think Doug and yossarian both touched on the main points, but to summarize, there are four reasons why you might not want to use a meat thermometer for candy:


Range A meat thermometer might go from 140 F to 220 F or something like that, which is plenty for meat. Candy often requires a range from about 75 degrees (chocolate) up to 400+ degrees (hard candy). The range on most thermometers is simply not sufficient for most candies.
Accuracy When you temper chocolate, 88 F degrees is an ideal working temperature (for dark chocolate). Some people would consider 91 so high you might want to think about starting over. When you make caramels, the difference between 235 F and 240 F can be the difference between wonderfully chewy caramels and a sugar-flavored rock. If you can't read one degree increments at a glance, you need a real candy thermometer.
Speed Meat thermometers often take 30 seconds to a minute to get an accurate result. When you are making candies, you have to be able to tell what the temperature is right now.
Contamination I don't want the thermometer that goes in semi-raw meat anywhere near my chocolate.