Laptops

Laptops with screens typically less than 12 inches diagonally and a weight of less than 1.7kg. Their keyboards are usually not full-size. Their primary audience is usually business travelers, who need small, light laptops. Ultraportables are often very expensive, have extended battery and/or battery life, house power-saving CPUs and almost always have integrated graphics.

Thin-and-lights

Laptops usually weighing in between 1.8kg and 2.8kg with a screen size of between 12 and 14 inches diagonally. Examples of this variety: the Sony VAIO FJ, Apple MacBook and Dell XPS M1210.

Medium-sized laptops

These usually have screens of 14 - 15.4 inches diagonally and a weight of around 3-3.5kg. They usually sacrifice a little computing power for smaller dimensions and longer battery life, although the length and width are usually determined by the screen size.

Desktop replacement computers

Powerful laptops meant to be mainly used in a fixed location and infrequently carried out due to their weight and size; the latter provides more space for powerful components and a big screen, usually measuring 17-20 inches. Desktop replacements tend to have limited battery life, rarely exceeding three hours, because the hardware is not optimized for efficient power usage.

Related devices

Laptops can be understood as a particular point on the continuum of more or less portable computing devices: the point at which the device is large enough to use substantially the same software as a desktop machine, but small enough to be support Mobile computing. Other points on the continuum include:

Transportables

Computers which can easily be moved from place to place, but cannot be used while in transit, usually because they require AC power. The most famous example is the Osborne 1. A transportable, like a laptop, can run desktop software; but it does not support mobile computing.

Tablets

Computers shaped like slates or (paper) notebooks, with touchscreen interfaces. As of 2007, the most common subcategory is the Tablet PC, which is essentially a laptop with a touchscreen. Some tablets have no keyboard; others, called "convertibles", have a keyboard which can be folded behind the screen. A tablet supports mobile computing, and, commonly, can run some desktop software (possibly with modification), but not all.

Ultra Mobile PCs (UMPCs)

Very small Tablet PCs.

Internet tablets

Internet appliances in tablet form. An internet tablet supports mobile computing, but usually does not run any desktop software.

Personal digital assistants (PDAs)

Small computers, usually pocket-sized, usually with limited functionality. A PDA supports mobile computing, but almost never runs any desktop software.

Handheld computers

A high-end PDA or small tablet.

Smartphone

A handheld or PDA with an integrated cellphone.

As will be clear, these categories are blurry at times. For example, the OQO UPC is a PDA-sized Tablet PC; the Apple eMate had the clamshell form factor of a laptop, but ran PDA software. The HP Omnibook line of laptops included some devices small enough to be called handheld computers. The hardware of the Nokia 770 Internet tablet is essentially the same as that of a PDA such as the Zaurus 6000; the only reason it's not called a PDA is that it doesn't have PIM software. On the other hand, both the 770 and the Zaurus can run some desktop Linux software, usually with modifications.