From what I know a stock is 8 x 10 base that you will have to cut down to fit. You pick a color from color wheel (you can send in a hair sample and they can get it close for you), stocks are usually freestyle with a body wave, density is 65%. You can get one of these very quickly from BA.

A custom is like anything else...custom fit to your liking. You send in template and get the exact size. Send in hair sample and get the exact color. You pick density, base, ventilation, curl and anything else you want to add. This normally takes 6-8 weeks to get.

I know BA may have more stocks to choose from now so you may want to send him an email.