The terms laptop and notebook are often used interchangeably to refer to any mobile computer, and not everyone agrees on what the differences are between these two terms, or if any exist. The conventional difference is that notebooks were so-named to differentiate this class of mobiles as ultra-light, ultra-thin, and highly portable, compared to original laptops that were comparatively cumbersome. With the advancement of mobile technology, laptops have evolved into what was previous considered a notebook, explaining why the terms have become interchangeable.
Today's ultra-light mobile computers that retain full operating systems and functionality are referred to as subnotebooks or ultraportables. Some models do not include a CD/DVD player, and they can be used with an optional docking station to enhance features. The specifications that qualify a notebook to fall into this category can be somewhat arbitrary but in general, the machine will be significantly smaller and lighter than the average laptop. High-quality, power-efficient components also yield longer periods between battery charges. These favorable attributes make ultraportables two to three times more expensive than their larger, heavier counterparts.
Mid-class mobile computers generally weigh close to 6 pounds (2.72 kg) with screens 15-17 inches (about 38-43 cm) measured diagonally. Keyboards are roomy, and the included DVD player might have Blu-ray™ capability. USB and Firewire® ports, built-in WiFi®, Ethernet, Bluetooth®, a Webcam, and a flash card reader have become standard in all but a few of the most affordable machines. Computers in this class have essentially become desktop replacements, blurring yet another distinction previously made to classify desktop mobiles as extremely heavy models geared towards features rather than portability, though some laptops are weightier.
At the lighter end of the mid-class line are standard mobiles that balance the features of mid-class against against the portability of subnotebooks. The standard laptop might have a 14-inch (35.56 cm) LCD and weigh in at 5 pounds (2.27 kg), though these parameters are only generally representative.
Netbooks (versus notebooks) are the smallest and most portable class, providing a low-priced alternative for online surfing and basic applications. They do not come with a CD/DVD drive, but they commonly include built-in WiFi®, Ethernet, USB ports, a flash card reader, a Webcam, and headphone and microphone jacks. Most displays are 6 to 10 inches (15.24 to 25.40 cm).
Within each of these classes — ultraportables, standard, mid-class and netbooks — there are models that fall between adjacent classes. An ultraportable at the heavier end of its class, for example, or a netbook at the larger and beefier end of its class might both be considered standard laptops by some. Models at the boundaries of classifications might be marketed as the more desirable classification. There are also additional classifications, such as rugged laptops for military or field use and the ultra-small subnetbooks, which can feature displays as small as 2.8 inches (7.11 cm).