Sugar – the difference between rock sugar and raw sugar

It's going to depend greatly on what you're baking. Sugar serves several different purposes beyond just providing sweetness. Besides sweetness:

• tenderness by interrupting and minimizing gluten formation. Sugar promotes spread in cookies

• Retain moisture and extend keeping quality (in baking sugar is actually considered a "liquid" ingredient due to its hygroscopic qualities - the ability to pull moisture from it's surrounding atmosphere).

• Promotes browning and caramelizing

• Assists in aeration and leavening (as in creaming butter and sugar to aerate the dough)

• stabilizes egg whites

• Provides food for yeast growth and fermentation

In some instances you might be able to use a syrup (honey, corn syrup, molasses, etc.) but not in all cases. For instance the granular nature of sugar is necessary for aeration of cookies and cakes because the jagged edges create air pockets as they pass through the fat.

Syrups primarily serve the purposes of sweetening, browning, and moistening. Honey could be used in muffins that are being made using the muffin method (aka two-bowl method) because this method would use a liquid fat (melted butter/oil) but not in the creaming method (producing a more cakelike structure from the creaming process). When using syrups you have to account for the addtional moisture that they provide. From: "How Baking Works" (Paula Figoni) "The National Honey Board recommneds the substitution for using honey in place of granulated sugar. This accounts for both the amount of water in honey and for its intense sweetness: use 1 pound honey in place of 1 pound granulated sugar; reduce water (or other liquid) in the formula (recipe) by 2.5-3 ounces."

Overall, when making substitutions of ingredients that are critical to the structural and eating qualities you probably will not be able to replicate the same results with the substitution. In the end, it will often be a case of "what is the next best thing" and realizing there will be quality differences in the finished product.

What Rincewind42 says about caramelization is true in itself, but it doesn't apply here. Cotton candy is made from sugar syrup at the hard ball stage (130°C), so too cold for caramelization. Caramelization only starts occuring at 160°C and above. In fact, caramel has a very different texture from sugar, so it is practically impossible to spin cotton out of it. It makes spidery, unendly elastic strands, unlike the brittle cotton candy strands. Plus, it is visibly brown, and tastes very different from sugar, so you'd notice it when eating the cotton candy.

The flavor of cotton candy is indeed sugar (unless it has had something else added). But taste is not the same thing as flavor. Things taste different to us based on texture, aroma, color and food name. (I once read about a study made with unusually colored puddings. People who had eaten cherry pudding with yellow coloring were sure they are eating banana or lemon, and they were convinced the pink vanilla pudding was strawberry). So it is entirely possible that a gelato customer who eats a gelato called "cotton candy", has lots of sugar and is painted a pastel color will perceive it as having a different taste from non-flavored ice cream which has lots of sugar, but not the color or the name, especially if the texture of the "cotton candy" flavor is tweaked a bit.