Wikipedia says this
A laptop computer (also known as notebook computer) is a small mobile personal computer, usually weighing around from 1 to 3 kilograms (2 to 7 pounds). Notebooks smaller than an A4 sheet of paper and weighing around 1 kg are sometimes called sub-notebooks and those weighing around 5 kg a desknote (desktop/notebook). Computers larger than PDAs but smaller than notebooks are also sometimes called "palmtops". Laptops usually run on batteries, but also from adapters which also charge the battery using mains electricity. Studies have been conducted to determine if the use of laptops on a man's lap causes harm to his reproductive ability (due to heat). The results are, thus far, inconclusive.
Laptops are capable of many of the same tasks that desktop computers perform, although they are typically less powerful. Laptops contain components that are similar to those in their desktop counterparts and perform the same functions but are miniaturized and optimized for mobile use and efficient power consumption. Laptops usually have LCD displays and smaller SO-DIMM (Small Outline DIMM) chips for their RAM. 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 mouse or keyboard can usually be attached.