Quick Jump
Digital Storefronts
Consider the difference between "existing on a larger site that advertises for you but you must bend to their whims" (Etsy, RedBubble, etc), and "having your own site you have to advertise yourself" (most others). People often have an Etsy to find new people, and then send a business card that gives their personal storefront info to order from in the future.
Art & Photo Prints Only
Print on Demand Shopfronts
These are your RedBubbles. You upload art, sometimes on a few different templates, and upload it. The website will do all the work advertising it, printing it, and shipping it.
Print on Demand Services
These services will take your art and print it on different items, and either ship it back to you or to your client. The main difference is that they do not have a shopfront for you and do not advertise for you. You use your own storefront.