The term "laptop" can refer to a number of classes of small portable computers
Full-size Laptop: A laptop large enough to accommodate a "full-size" keyboard (a keyboard with the minimum QWERTY key layout, which is at least 13.5 keys across that are on ¾ (0.750) inch centers, plus some room on both ends for the case). The measurement of at least 11 inches across has been suggested as the threshold for this class.[16] The first laptops were the size of a standard U.S. "A size" notebook sheet of paper (8.5 × 11 inches)[citation needed], but later "A4-size" laptops were introduced, which were the width of a standard ISO 216 A4 sheet of paper (297 mm, or about 11.7 inches), and added a vertical column of keys to the right and wider screens.
Netbook: A smaller, lighter, more portable laptop. It is also usually cheaper than a full-size laptop, but has fewer features and less computing power. Smaller keyboards can be more difficult to operate. There is no sharp line of demarcation between netbooks and inexpensive small laptops; some 11.6" models are marketed as netbooks. Since netbook laptops are quite small in size, CDs cannot be used in these computers.
Tablet PC: these have touch screens. There are "convertible tablets" with a full keyboard where the screen rotates to be used atop the keyboard, and "slate" form-factor machines which are usually touch-screen only (although a few older models feature very small keyboards along the sides of the screen.)
Rugged: Engineered to operate in tough conditions (mechanical shocks, extreme temperatures, wet and dusty environments).
A modern laptop computer
A desktop-replacement computer is a laptop that provides all of the capabilities of a desktop computer, with a similar level of performance. Desktop replacements are usually larger and heavier than standard laptops. They contain more powerful components and have a 15" or larger display.[15] They are bulkier and not as portable as other laptops, and their operation time on batteries is typically shorter; they are intended to be used as compact and transportable alternatives to a desktop computer.[15]
Some laptops in this class use a limited range of desktop components to provide better performance for the same price at the expense of battery life; a few of those models have no battery. [17]
In the early 2000s, desktops were more powerful, easier to upgrade, and much cheaper than laptops, but in later years laptops have become much cheaper and more powerful,[18] and most peripherals are available in laptop-compatible USB versions which minimise the need for internal add-on cards. In the second half of 2008, laptops outsold desktops for the first time.
Source : http://en.wikipedia.org/wiki/Laptop