In an all-in-one, the computer proper is inside a flat screen, meaning there is no separate tower or system unit. There is a conventional keyboard and mouse, which may be wireless. Wi-Fi is likely to be built in, with the consequence that often there is only a single cable for the entire system, in the form of the power lead.
The genre was largely invented by Apple, which leads the pack in terms of style and elegance. But such sophistication comes at a price - even the cheapest model is an eye-watering £899 (see http://ctacs.weebly.com/news--articles/new-cheap-computer-from-apple-just-899). However, the manufacturers of Windows computers are in on the act, too, with models available from Dell, HP, Asus, Acer and others.
Lenovo are another big player and have a wide range of models. They have been helping make the genre more affordable, by offering a variety of entry models through outlets such as Currys/PCWorld and Argos. For instance, the C260 is available in a choice of colours (okay, black or white) for just £299. This buys a system with a dual-core Intel processor, 4GB RAM, 500 GB hard drive and a 19.5" screen with 1600x900 resolution. By modern standards it is not a speed demon, but is more than adequate for most purposes. In fact it is actually quite nice, quite compact and well suited to a domestic environment.