A one piece (closed, drainable or urostomy) is where the adhesive baseplate and the actual pouch are all together as one.
A two piece on the other hand, is completely separate pieces: a baseplate and a pouch. The adhesive baseplate is positioned first and then the pouch (closed, drainable or urostomy) is attached to the baseplate using an adhesive or mechanical system. The mechanical system has a baseplate with a plastic ring that the pouch will clip on to. The adhesive system has a baseplate with a plastic area that the pouch will stick on to. Some two piece use a combination of both, clip on and stick on. The benefit of using a two piece is that the baseplate is designed to stay on the skin for several days at a time and the pouch can be changed as appropriate.
Your Stomal Therapy Nurse will guide you to select the pouching system which best suits your needs, especially in the beginning as you learn how to care for your stoma.