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Desktop replacement

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.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.
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. 
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,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.
The names "Media Center Laptops" and "Gaming Laptops" are used to describe specialized notebook computers.

 Subnotebook

A subnotebook or ultraportable, is a laptop designed and marketed with an emphasis on portability (small size, low weight and often longer battery life) that retains performance close to that of a standard notebook. Subnotebooks are usually smaller and lighter than standard laptops, weighing between 0.8 and 2 kg (2 to 5 pounds);the battery life can exceed 10 hours when a large battery or an additional battery pack is installed. Since the introduction of netbooks, the line between subnotebooks and higher-end netbooks has been substantially blurred.
To achieve the size and weight reductions, ultraportables use 13" and smaller screens (down to 6.4"), have relatively few ports (but in any case include two or more USB ports), employ expensive components designed for minimal size and best power efficiency, and utilize advanced materials and construction methods. Most subnotebooks achieve a further portability improvement by omitting an optical/removable media drive; in this case they may be paired with a docking station that contains the drive and optionally more ports or an additional battery.
The term "subnotebook" is reserved to laptops that run general-purpose desktop operating systems such as Windows, Linux or Mac OS X, rather than specialized software such as Windows CE, Palm OS or Internet Tablet OS.
At Computex 2011 Intel announced a new class for ultraportables called ultrabooks. The term is used to describe a highly portable laptop that has strict limits for size, weight, battery life, and have tablet-like features such as instant on functionality. Intel estimates that by the end of 2012, 40 percent of the consumer laptop market segment will be Ultrabooks. 

Netbook

Netbooks are laptops that are light-weight, economical, energy-efficient and especially suited for wireless communication and Internet access. Hence the name netbook (as "the device excels in web-based computing performance").
With primary focus given to web browsing and e-mailing, netbooks are intended to "rely heavily on the Internet for remote access to web-based applications" and are targeted increasingly at cloud computing users who rely on servers and require a less powerful client computer. A common distinguishing feature is the lack of optical disk (i.e. CD, DVD or BluRay) drives. While the devices range in size from below 5 inches to over 12, most are between 9 and 11 inches (280 mm) and weigh between 0.9–1.4 kg (2–3 pounds).
Netbooks are mostly sold with light-weight operating systems such as Linux, Windows XP and Windows 7 Starter edition.
Because they're very portable, Netbooks have a few disadvantages. Because the netbooks are thin, the first such products introduced to the market had their primary internal storage in the form of solid-state drives and not hard disks, which are essential to installing very many programs. Hard disk drive technology and form factors have since been adapted to fit into netbooks.
Given their size and use of more rudimentary components compared to notebooks and subnotebooks, netbooks also generally have a smaller-capacity hard drive, slower CPU, and a lower-profile RAM capacity.
Recently, Google has announced to be developing an own operating system called Chrome for this market.
The big breakthrough for netbook computers did not happen until the weight, diagonal form-factor and price combination of < 1 kg, < 9", < U.S. $400, respectively, became commercially available at around 2008.