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A tablet computer, or simply tablet, is a one-piece mobile computer. Devices typically offer a touchscreen, with finger (or stylus) gestures acting as the primary means of control, though often supplemented by the use of one or more physical context sensitive buttons or the input from one or more accelerometers; an on-screen, hideable virtual keyboard is generally offered as the principal means of data input. Available in a variety of sizes, tablets customarily offer a screen diagonal greater than 7" (18 cm), differentiating themselves through size from functionally similar smart phones or personal digital assistants.
Though generally self-contained a tablet computer may be connected to a physical keyboard (or other input device), and a number of Hybrids that offer a detachable keyboard have been marketed since the mid 1990s, as have a number of convertible touchscreen notebook computers that offer an integrated keyboard that can be hidden by a swivel joint or slide joint, exposing only the screen for touch operation. Tablets have also appeared in a fold-able Booklet format that offer the user dual-touchscreens, and can be used as a notebook by displaying a virtual keyboard on one of the displays.
Conceptualized in the mid 20th century and prototyped and developed in the last two decades of that century the devices only became practical and affordable in the early years of the present century.
As of March 2012, 31% of U.S. Internet users were reported to have a tablet, which was used mainly for viewing published content such as video and news. Among tablets available in the market in 2012, the top-selling line of devices is Apple's iPad with 100 million sold by mid October 2012 since it was released in April 3, 2010, followed by Amazon's Kindle Fire with 7 million, and Barnes & Noble's Nook with 5 million.

http://en.wikipedia.org/wiki/Tablet_computer