Laptops
In one sense the generic term laptop still applies to a portable computer with a fold-down screen and the ability to run on a rechargeable battery. But increasingly it tends to mean a computer of this variety that doesn't have anything special about it (size or weight) to differentiate it into one of the new categories. It will have a full size keyboard and a screen measuring between 13 and 17 inches, of which the most common variant is 15.6 inches. Such laptops have become very cheap, with models readily available for under £300. Even at this price point it is not unusual to get a generous amount of memory (4GB) and a capacious hard disk (320GB or 500GB). However, cheaper models tend to have low performance processors and it is worth paying extra for a better one (Intel Core i or an AMD quad-core is a good choice of processor). A laptop with a 17" screen makes a good alternative to a standard desktop for many people; it may not be particularly portable but it takes up less space and has less wires.
Netbooks and Netbooks Redux
A netbook is a very small laptop, typically with a 10.1" screen. They first appeared 4 or 5 years ago and quickly set the world alight... for all of about 10 minutes. The reasons they caught people's imaginations were that they were small and light and very affordable. However, there were compromises: they used slow processors, had small keyboards and small low-resolution screens. Additionally, to keep the price down many were made of cheap plastic. The craze didn't last long and the relative demise of the netbook was hastened by the arrival of the iPad, which for many people offers a far more usable device for when they are on the move. Netbooks are still available, but are not big sellers.
But, a new variant may be resuscitating the genre. It doesn't have a name so we'll call it Netbook Redux (remember, you heard it here first!). These machines have larger screens with higher resolution, usually 11.6" with 1366x768 resolution and are LED-backlit, making them brighter and slimmer. Keyboards are larger, often with nearly full-size keys. The processors, usually made by AMD, are not quite so anaemic and usually have reasonable graphics capabilities, such that they can play back HD video without problems. Acer, Samsung and Lenovo are the main vendors - expect to pay around £300 depending on specification.
Ultrabooks
Apple was the one vendor that didn't launch a netbook. Instead, in 2008 it launched the MacBook Air, which at a stroke redefined what an ultraportable laptop should be. It was thinner, lighter and considerably better than anything else and has continued to evolve ever since. Ultrabook - a specification from chip manufacturer Intel - is an attempt to bring those sort of capabilities and flare to the wider Windows world. Ultrabook laptops are comparatively thin, have good processors, superior design and long battery lives when compared to 'normal' laptops.
The first generation of Ultrabooks were a flop, largely because they were too expensive. Many were priced around £1000; quite frankly, anyone who is spending this much on a very portable laptop is going to be inclined to buy the real thing i.e. a MacBook Air. However, Intel and the vendors have evolved the specification and learnt. It is now possible to buy Ultrabooks with screens from 11.6" to 15.6", whilst prices have fallen and budget models have appeared. A good example would be the Lenovo U310, which costs £599 with a 13" screen and (in the opinion of many) is a dead-ringer for the MacBook Air.
It is likely that prices for Ultrabooks will continue to fall. The Netbook Redux described above can be thought of as a poor man's Ultrabook, and it is possible that these two categories will merge at the economy end of the market.
Chromebooks
The buzzword these days is 'cloud computing' and the Chromebook is a laptop specifically designed to operate in the online world. There is no local storage in the conventional sense; rather, all your work is stored on Google's cloud (the Chromebook specification is from Google). Nor are there any applications stored locally. Effectively, all the Chromebook has is a browser (yes, it is Google Chrome!) with icons corresponding to applications in the cloud. There are some huge advantages to doing this: applications do not have to be installed, everything is always up-to-date, there is no risk of a virus or other malware. However, the obvious problem: what happens when the internet is not available?
Although they are brand new, it is possible that the Chromebook is already dead in the water. The overall approach is probably correct - things are moving to the cloud - but most people will prefer these sort of features to be grafted on to more conventional computers (which is happening e.g. with the integration of SkyDrive with Windows 8 and iCloud with the Mac OS).
Conclusion
Too much choice can be a bad thing! But, if you want the most value for money, buy a conventional laptop. Look to spend £400 rather than £300 as you'll get a lot more oomph. If you want a very portable device and are happy to spend the money, buy an Ultrabook but keep an eye out for falling prices, else go for a Netbook Redux. Conventional netbooks and the new Chromebooks may not be suitable purchases for most people.