The most popular categories of computers are: personal computers, handheld computers, workstations, server computers and supercomputers.

Personal Computers (PCs) define computers designed for general use by a single person. They come in a variety of types, such as desktop, notebook and wearable PCs, Notebook PCs, also called laptop PC,are called, compact and portable. They integrate the display, keyboard, a pointing device or trackball, processor, memory and hard drive all in a battery operated package slightly larger than an average book.
Desktop PCs are set in a permanent location, not designed for portability. The various input/output components of a desktop PC are connected to its system unit, a box which contains the processor, disk storage and other electronic component. Wearable PCs are worn by the user, about the waist, head, or on the arm. To creal truly 'personal' computers, designers have divided the components into cable-connected modules. The components are very light, covered in soft plastic and strapped on with Velcro.

Handheld computers can be held in your hand. They are very light, can operate for days on their batteries and can fit in a pocket or a handbag. The only problem is the keyboard is miniaturized, making data entry and interaction whit the computer is difficult and slow. Handheld computers and person digital assistants (PDAs), also called palmtops, depend on touch screen technology for user input.
ESERCIZIO
You can hold in your hand a PDA.
Computer designed for the use of a single person are called personal computers.
When you look for a computer which is portable and flexible you choose a notebook PC.
In many jobs it's important to have hands free, so they use wearable PC.
Handheld computers, give people access to important business and personal information at any time, at any place.

Workstations look like a desktop PC but their characteristic is that they are very fast. Workstations are for "power users' - engineers doing design work, scientists and researchers, graphics designers, creators of special effects for movies, and so on. Another difference between workstations and desktop PCs is represented by input/output devices. A typical workstation will have: at least one large-screen colour monitor capable of displaying photo-like images; a variety of spedacilised point-and-draw devices that combine high precision and easy use; add-on keypads to expand the number of speciatised function keys. The future? In a few years, the average PC will have the capabilities of today's workstation.

Server computers. Most computers, including PCs and workstations, exist as part of a network of computers. The server computer links and provides supporof the client computers on the network. Typically, the client computer is a PC or a workstation. The server computer (also called mainframe) is a large, powerful computer which is capable of supporting large numbers of terminals (typically 500+). Banks, building societies, the utility companies (for example, gas, electricity, water) and e-commerce firms on the Internet need more power than an ordinary computer can prowde. So they use mainframes or servers.

Supercomputers are very tast computers that were created to respond to the needs of scientific jobs invotving the manipulation of a complex mathematical model that may require trillions of operations to resolve. Supercomputers are famous for their apptications that are highty complex, require a vast number of catcutatio and very littte input/output. For example, IBM's blue Gene is a supercomputer with over a million processors that can perform 1 million billion (10^15) math operations per second. Som examples of applications:
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•The simulation of airflow around an airplane at different speeds and altitudes.
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•Studying how oceans and the atmosphere interact to produce weather phenomena, such as El Nino.
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•Creating the advanced graphics used to create special effects for Hollywood movies such as The Lords of the Rings Trilogy.



• Computers extract information from the world around them by using devices such as microphones and photosensitive diodes to detect changes in energy levels. In both devices the energy (from sound waves or light) converts into an electrical current. The current is in an analog form, made of continuous changing voltages.  Computers are designed to manipulate digital data- specific values are expressed in real numbers. A computer must convert all analog input it receives into digital values before the PC can use it. Viceversa, many of the results of the computer's digital maniputation must be converted back to analog when it becomes output.

Analogue to digital conversion and digital to analogue
An electronic device is digital if data in it is represented as electrical ,on' and ,off' signals that correspond to binary digits and can be stored in computer memory. The data is therefore represented as a succession of 1s and 0s.
A device is analogue where data is represented as signals that vary within a predefined range. Traditional watch faces are analogue. The hands move continuously round a dial, within the predefined range of set seconds (0 to 60), minutes (0 to 60) and hours (12 or 24 in totlt). Time is represented by the position of the hands on the dial.
Many modern watches have a digital display, in which time is represented by the digits shown on a small display screen.
Digital signals have two advantages:
•• they can be copied exactly, without even the stightest toss of quatity
•they can be further processed by computer.

All modern, general-purpose computers are digitat, but analogue computer circuits are used in industrial control equipment.

A digital computer is more accurate than an analogue computer because it only needs to sense the difference between clearty distinguishabte states. For example,a slight fluctuation in electrical voltage would affect the result in an analougue computer but would not affect a digital computer because it coutd still easily distinguish, the 1 state from the 0 state of any circuit element. For the same reason digital music reproduction (as on a CD) is more accurate than analogue reproduction (on a traditional vinyl record or cassette tape)

Digital is the wave of the future because it gives us an accurate way to rapresent data and an easy way to store and manipulate that data. Two other advantages of digital technology are the recording does not degrade over time and the digitized information can often be compressed. To conclude, we can only say that the move into digital world is inevitable.

Cellular phones and BlackBerry
One of the most effective use of the digitat versus analog technology is in the booming cettular market. With new phone activations increasing exponentially,the limits of analog are more and more evident. With the all-in-one phones out now, digitat phones offer more features than their analog predecessors.

Think of the BlackBerry, for example. It is widety used by enterprises, business men and potiticians (like Obama) especiatty because of its wireless email handling capabiiity. The BlackBerry was firstly introduced in 1999. In 2002 commonly known smartphone BlackBerry was released, which supports push e-mail, mobile telephone, text messaging, internet faxing, web browsing and other wireless information services as well as a multi-touch interface. In 2007 the number of BlackBerry subscribers reached 8 million.

General-purpose vs. special-purpose computers
Most spedal-purpose computers are used to control things. They exist as tiny chips, embedded in some device, which help the device to operate smoothly.
Frequently, they control timing (alarms and spark plugs), temperature (thermostats, dishwashers), digital readouts (speedometers, scales, and watches), various sensors (car door closed or seat belt fastened), and diagnostic devices (echocardiogram machines and fuel level indicators). Each special-purpose computer performs only the task it was built to do in that specific device.
A general-purpose computer, on the other hand, can do many things. For example the same general-purpose computer that performs word processing can be used to control an assembly line or to perform statistical analysis of numerical information. The desktop or laptop computer we use today is a general-purpose computer of extraordinary flexibility, capable of performing numerous different tasks, often simultaneously. Its computational power might be compared to that of larger commerciatty available machines, lacking onty the multipte processors that give supercomputers their legendary speed and power. Today, personal computers and workstations can do anything supercomputers do, only more slowly.