Oh where to begin...?
I guess I'll start with a quick breakdown of the different types of handcrafted soaps. There's those that are made from scratch by the artisan using the cold or hot process methods. Cold process allows the crafter to make a smoother soap, with more intricate designs. Hot process soap is more rustic in appearance, but offers the maker some advantages over cold process. These soaps need to undergo a curing phase for optimum performance. Depending on the fats used, this can be a month to a year. Then there are commercial soap bases. Most of these bases are referred to as "melt and pour" soap because, well, you melt it, add fragrance and colorants if desired, and pour it into a mold. These have the advantage of being ready for use the same day and also doesn't require the soaper to handle lye. I also like that many of the bases are transparent, which allows for the light to catch glitters and micas that are suspended in the soap. I have a book that explains how to make transparent soap yourself, but I've yet to venture down that road.
Now I'll back up a bit and define soap itself. It's a chemical reaction between fats, water, and lye called saponification. Once this reaction is complete, the lye is gone and you're left with soap and glycerin. Glycerin is a humectant, which draws moisture to your skin and holds it in. Sadly, most commercial soap makers remove the glycerin and sell it. What a shame.
Have I mentioned I'm a science nerd? I'll spare you from any more of it, for now;-)