An all in one computer is a desktop computer that has all its components built into a single display box. In other words, the monitor of an all in one PC also contains the CPU, speakers, mic, webcam, DVD drive etc there by making its construction compact. The only external peripherals for such a PC is usually a wireless keyboard and mouse.
Though the hype around all-in-one desktops is more now, the fact is that they have been around for almost 25 years now with Apple being the pioneer in this technology. They became much more popular now because the erstwhile fat CRT monitors have been replaced with thinner and flat LCD monitors now and hence compact casing is possible.
The main advantage of an all in one computer is that it occupies a lot lesser space than a desktop computer.
Most of the all in one PCs of today are elegant units and there is a style factor and pride attached to the ownership of the same.
Also, since the number of cables are lot lesser, the electric wiring and plug point need is less too. Needless to add, fewer wires mean less mess.
Modern all-in-one PCs also boast multi touch display taking cues from the tablets thereby making their user experience much more enjoyable. Check out this HP TouchSmart All-in-One, for example.
Yet another advantage of all-in-ones is that they are easy to relocate as they usually have a handle to carry them around.
All-in-One desktops though look cool, have a few disadvantages attached to them too. The main disadvantage is the pricing itself. Most all in one PCs are costlier than their regular desktop counterparts where as they are less powerful than desktop PCs in terms of processing speed, graphics capabilities etc.
Upgrading and customizing options for all-in-one PCs are usually limited and mainly restricted to RAM upgrade etc. Also, it is not easy to tweak such computers and hence they are not a good choice for those who like to tweak and overclock their computers. Recent all in one PCs, though, have started focusing on the upgradability aspects.
Self-repairing an all-in-one PC is close to impossible where as a regular desktop is a lot more modularized and with some experience you can replace the damaged parts yourself. At this point, I should also mention that the spares and service cost may be on the higher side for all-in-one desktops.
At the end, it is one’s convenience, needs, space availability and affordability that drive an all-in-one PC purchase decision.