The word "laptop" can either be used as a noun or adjective. As a noun, it means something not too entirely dissimilar to "a small computer having its components (typically processor, keyboard, and display screen, and some sort of cursor manipulation device) in a portable case capable of independent, battery-powered operation". As an adjective, it is something more along the lines of "something that is easily used or operated (due to size, shape, and design) in one's lap". In this, my focus shall be on the noun form of the word.

The origins of the laptops are a bit blurry (I have found 6 sources that say different computers were the "first laptop computer"), but the reasons for this is clear. Portability (obviously). Computer popularity, and usefulness, increased a great deal in the 80's. Much like the introduction of oil paints to art, the laptop allowed people to move with their tools of business, education, and entertainment to other venues. No matter if the first laptop was the "Grid Compass" (a small portable computer used by NASA on shuttle missions that had a small die-cast magnesium case) or "Dynabook" (a notebook computer that xerox researched in the 1970s), data on computers could be easily transported and even created nearly anywhere.

A trend with technology has been that not only has power of computer components increase, but size has decreased. This allows for such components to be easily put into portable units. There are 6 main components to a laptop: Microprocessor, Memory, Storage Devices, Power Supply, Input Devices, and Display.
  • Microprocessor - This is ANY computer's "brain". It executes all functions of the unit. Laptops tent to have processors that use less power and generate less heat than those of desktops. This is obviously done to conserve batter life. Laptops may run at lower speeds or use less voltage to accomplish this. Despite a blow to battery life, many laptops have been including desktop grade processors.
  • Memory - Again, every computer has memory. Both RAM and ROM (imagine that). The ROM, like desktops', contains BIOS. The RAM, yet again like desktops', is used to temporarily store data. Although RAM does the same thing in both, the laptop version is proportioned differently to be more portable and durable. Memory tends to be less expandable in laptops.
  • Storage Devices - They store data on a less temporary level. All laptops should include a hard drive. The hard drive stores large amounts of data (modernly several gigabytes). These hard disks are almost always smaller than those of desktops models due to size and power limitations. Floppy disk drives are not always include in laptops. The format is beginning to slowly fade out of common use and stores too little data to take up so much room in a unit. External floppy drives may supplement the lack of one in the case. CD and DVD drives are preferred over floppy drives due higher capacity and efficiency. Interchangeable drives can be either "cold-swappable" (where the computer mused be powered off and the exchanges) or "hot-swappable" (where they can be exchanged while on).
  • Power Supply - Laptops today use a combination of battery and wall power. When plugged into walls, power is "unlimited" (I mean it's got to end some where). During this time the power coming from the socket will also charge the battery in the laptop. Battery life is a major concern with laptops (what is the point in having a unit that won't let you spend more than 30 minutes away from a power source?). Laptops almost always include an internal battery that is typically lithium, nickel-cadmium, or nickel-metal hydride. External supplements are often offered to extend battery life. These batteries' power, like all of technology, increase with time, but in many cases their lives do not extend as much. While they are getting better, demands of other components within the laptop may strain battery life making the change seem less apparent.
  • Input Devices - Because a mouse would take up too much room and may be hard to use in many cases, it typically has been replaced in laptops by one of the following: a trackball,trackpoint, or touchpad. These devices may not be operated with as much ease as a mouse, but they are quickly adapted to. Along with one of these, a laptop features a compact version of a keyboard built into the case.
  • Display - Laptops today feature LCD screens. These have evolved from grayscale to true-color images. Some are backlit, some are reflective. External monitors or data projectors can be used with laptops to easily present data to a group or put less strain of eyes.
Laptops have came from weighing up to 26 pounds to as low as 3 pounds. They can be seen anywhere from the local coffee house, to special terminals for laptops at airports, to the International Space Station. They talk to eachother wirelessly and can be seen all around us. They are always evolving to be bigger better, faster, and live longer. We humans should be glad they are not graced with free will!
http://inventors.about.com/library/inventors/bllaptop.htm
http://biz.howstuffworks.com/laptop1.htm