An AIO (desktop computer) has the motherboard in back
of the LCD monitor panel, or it's possible for a "base unit",
a brick, to be located below the monitor, and have some
of the electronics in there. The PSU and disk drive could
go in the base for example.
Some AIO even have an optical drive, and you put the optical
discs in vertically.
It's just a laptop, but without a keyboard. It means
your keyboard is mobile, and the keyboard could be moved
out of the way, leaving the rest of it sitting there static.
Usually a bit heavier than a laptop, so does not
particularly encourage movement. Can also have a larger
screen than a laptop might have (Apple kind of defines the
archetype, and others salt to taste).
If you're lucky, it doesn't use eMMC. Leaving your
disk drive as a maintainable item.
Generally, unless your eyeballs are a limitation, a
laptop is a better usage of money. If you were to
get one with a yard-wide IPS panel, it could easily
cost a couple thousand bucks for no particular reason.
Laptops are pretty limited on screen options.
Paul