Sugar (sucrose) is a molecule made up of glucose and fructose. When dissolved in water, the process is known as saturation. More sugar can be saturated into the water at different temperatures which is why we heat the solution to have a higher concentration of sugar in a separated state. When the water eventually evaporates, you are left with a supersaturated solution which is in an unnatural state and hence unstable. This is why you must not stir the solution - to prevent crystallisation i.e. the fructose and glucose bonding again.
Molecules of sucrose are quite geometrical and when crystallised, the crystals are square - like building blocks. To help prevent crystallisation, an acid/fructose such as lemon juice or cream of tartar can be added before boiling, or a glucose solution. This makes the molecules odd shapes and harder to form geometrical solid blocks. So it helps keep them separate, enabling a clear toffee. But I like to just keep it simple:
Heat sugar with half the quantity of water over low heat. Stir until completely dissolved, taking care that there are no sugar crystals on the side of the pan. Once dissolved, boil until water evaporates and sugar caramelises. Pour over roasted nuts and you have praline!
To make fondant which requires a lot of crystallisation, sugar syrup is boiled, cooled and beaten for about 15 minutes – extremely cloudy toffee!