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This is a simple question and you can get the answer from
http://en.wikipedia.org/wiki/Laptop
A laptop computer, or simply laptop (also notebook computer or notebook), is a small mobile computer, which usually weighs 2.2-18 pounds (1-6 kilograms), depending on size, materials, and other factors.
While the terms laptop and notebook are often used interchangeably, "laptop" is the older term, introduced in 1983 with the Gavilan SC. "Notebook computer" is a later coinage, which was used to differentiate smaller devices such as those of the NEC UltraLite and Compaq LTE series in 1989, which were, in contrast to previous laptops, the approximate size of an A4 or letter size paper sheet. The terms are imprecise: due to heat and other issues, many laptops are inappropriate for use on one's lap, and most notebooks are not the size of typical letter or A4 paper notebook. Although some older portable computers, such as the Macintosh Portable and certain Zenith TurbosPort models, were sometimes described as "laptops", their size and weight were too great for this category.
As of 2007, most manufacturers use the term "notebook" (or some variant thereof) for what most end-users call a "laptop".
Laptops usually run on a single battery or from an external AC/DC adapter which can charge the battery while also supplying power to the computer itself.
As personal computers, laptops are capable of the same tasks as a desktop PC, although they are typically less powerful for the same price. They contain components that are similar to their desktop counterparts and perform the same functions, but are miniaturized and optimized for mobile use and efficient power consumption. Laptops usually have liquid crystal displays and most of them use different memory modules for their random access memory (RAM), for instance, SO-DIMM in lieu of the larger DIMMs. In addition to a built-in keyboard, they may utilize a touchpad (also known as a trackpad) or a pointing stick for input, though an external keyboard or mouse can usually be attached.
Many schools have taken in laptop based programs in which every student receives a laptop for school use only. The operating system is configured (or a third party program is installed) to limit the student's access to perform administrative tasks such as hardware or software installation, or operating system modifications. Students are generally permitted to use these systems to take notes, write papers, and perform other school-related activities.