A sugar or candy thermometer is used to measure the temperature and therefore the stage of a cooking sugar solution. The final texture of candy depends on the sugar concentration. As the syrup is heated, it boils, water evaporates, the sugar concentration increases, and the boiling point rises. A given temperature corresponds to a particular sugar concentration. In general, higher temperatures and greater sugar concentrations result in hard, brittle candies, and lower temperatures result in softer candies. The stages of sugar cooking are as follows. Wikipedia has excellent information about the temperatures of the different stages.