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".
Predecessors of the laptop include the Osborne 1 and the Macintosh Portable, each of which weighed 16-30 pounds (7 to 14 kg) (due in part to being powered by hefty lead acid batteries) but nonetheless offered novel mobile computing platforms. Laptops are generally popular among students, travellers, and telecommuters.
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 SODIMM (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.
Most modern laptops use an active matrix display, with screen sizes 14 inch (350 mm) or larger, and have PCMCIA expansion bays for expansion cards. Internal hard disks are smaller—2.5 inch (64 mm) compared to the standard desktop 3.5 inch (90 mm) drive—and have lower performance. Display adapters and sound cards are integrated. Modern laptops can often handle sophisticated games but tend to be limited by their fixed screen resolution and display adapter type. Docking stations may be used for expanding connectors and quickly connecting many components to the laptop.
Current models use LiIon batteries, which have largely replaced the older NiMH technology. Typical battery life for most laptops is two to five hours with light-duty use, but may drop to as little as one hour with intensive use. Batteries gradually degrade over time and eventually need to be replaced, commonly after two to five years.
Laptops generally cost around twice as much as a desktop machine of similar specification. Performance is always lower than that of a comparable desktop because of the compromises necessary to keep weight and power consumption low. Upgradability is severely limited: typically only the RAM and hard drive can be changed. Because nearly all functions are integrated into the proprietary-design mainboard to save space and power, laptops are difficult to repair economically. Outright replacement of faulty parts can include the display screen, drives, daughterboards, modem, storage devices and other components, but repair costs can be high, even when feasible.
However newer laptop computers have now been able to rival their desktops and the term desktop replacement is a perfect description of this new situation (sometimes referred to as a "desknote"). Thanks to the development of powerful batteries some desknote developers have now started placing desktop components directly into these computers which makes them equal in performance with their desktop counterparts of similar specifications. Consequently, these new desknotes are much larger than predecessors which would be expected as they are using the larger components of desktops. This is also where that term desktop replacement comes into play since these computers are also too large to carry around, most people who purchase these computers use them mostly at their desk and then carry them only on a rare basis. The performance gap between these computers and desktops has been shrinking thanks to the aggressive tactics of desknote developers who continually try to squeeze more desktop power into these laptops. However, laptops provide simple mobility which these new desktop replacements do not have, so standard laptop sales have stayed high regardless of the extra performance desktop replacements provide.
A further category of ultra-light laptops has developed, which emphasizes the small, light profile of the genre (screen size of 12 inch diagonal or less), while packing as much performance as possible into the package. These are called sub-notebooks or palmtops.