All-In-One computers are a display screen with the computer built into it’s frame. Very nice for space saving options as you don’t have a separate tower.
Small Form Factor computers are a miniature version of a desktop system. This lets you choose what size monitor display you want, but gives you a very small footprint for the actual system box.
Both AIO and SFF computers do have the same limitations of notebooks/laptops, in that due to their unique design and components, upgrade options are limited at best, and the unique parts for the systems can make getting replacement parts difficult