There are a few main categories of laptops, categorized based on price, size, and power:

1. Entry-level.  These laptops usually have Intel Celeron-M or AMD Mobile Sempron processors, integrated graphics, less RAM than other laptops, and small hard drives.  Price is the main concern here, so they will be as heavy as mainstream laptops but not nearly as powerful.  Be careful in this category, as most manufacturers cut corners on their entry-level models and reliability suffers.  Some manufacturers, such as Averatec and eMachines, actually specialize in entry-level machines.
   


2. Mainstream.  Mainstream laptops are what they sound like – middle-of-the-road.  They are fairly powerful, although not as much so as their desktop-replacement counterparts.  They weigh between 5.5 and 7.5 lbs, and usually have 14” or 15” regular-aspect or widescreen displays.  They probably have mobile processors, plenty of RAM and hard drive space, and may even have dedicated graphics.  They’re not huge, but you would probably want to keep them on a desk most of the time.  Most people choose this type of laptop as they are a nice compromise of all the different types.
   


3. Business.  Business laptops are similar to mainstream laptops, but are usually significantly more expensive.  However, the extra cost pays off in better construction and higher-quality components, with leads to increased reliability and durability.  Usually they are designed fairly conservatively, often with mostly-black casings, and often they offer more port connections, docking-station support, and better keyboards and pointing devices.  Sometimes they offer security devices such as fingerprint readers or smart cards to prevent unauthorized access.  They also tend to weigh slightly less than their mainstream counterparts, but are less performance-oriented.  Lenovo’s ThinkPad line and Dell’s Latitude line are the most-often-purchased business laptops.
   


4. Desktop replacement.  These are the huge laptops that are becoming increasingly popular with consumers.  They offer 17” widescreens, and in some cases we’re starting to see 19” and even 20” widescreen models.  They are big and heavy (at least 8 lbs, and more often around 11 to 13 lbs).  But more importantly, they are very powerful, with the best graphics cards and sometimes desktop processors.  Some also feature upgraded speakers, keyboards with built-in numeric keypads, and even multiple hard drives.  Obviously these laptops are shooting to be as powerful as desktops, but to take up less room and be more portable.  This category is dominated by boutique gaming manufacturers, but almost every major manufacturer has an entry here.
   


5. Thin-and-light/ultraportable.  These are usually more business-oriented models that are smaller than other types of laptops, with 11” to 13” screens, and weighing a maximum of five pounds.  They almost always have integrated graphics, and often come with low-voltage CPUs and high-capacity batteries.  The main objectives here are go-anywhere portability and all-day battery life.  Sony and Fujitsu are notably strong in these areas.
   


6. Tablet PC.  Tablets are a type of ultraportable laptop that have a touch-sensitive swiveling screen.  They usually are based on Windows XP Tablet PC Edition, which offers support for pen-based input, including handwriting recognition.  These are quite useful as a sort of digital notepad or overhead projector, and can be helpful for taking notes when you need to add in diagrams or equations in addition to the text-based notes.  Many laptop manufacturers, notably Fujitsu, Toshiba, Compaq, Acer and Lenovo, have tablets, but arguably they have been somewhat of a flop in the marketplace due to their high price and generally low processing power.