What is a Computer?

A computer is an electronic device that transforms or manipulates digital data to produce usable information by autonomously following a sequence of instructions.

A computer system is made up of hardware and software.

Hardware - tangible, physical components; e.g. chips, wires, disks, monitors, etc.  The core components are the processor and memory, the peripheral components are the keyboard, monitor, mouse, etc.
Software - intangible programs that provide directions to the hardware, and the data that is manipulated.

Simplified Conceptual Model of Computer Organization

Input devices:

• Input devices are machines that communicate with the computer.
• Examples of input devices are the keyboard and the mouse.
• Input devices are controlled by the user.
• For example, typing on the keyboard or clicking on a mouse button.

Processor:

• The processor is a microprocessor chip inside the computer.
• All processing and manipulation of data takes place in the processor.
• Processing is the "thinking" that the computer does - the calculations, comparisons, and decisions.
• Each chip manufacturer wires the circuitry with a unique "instruction set" thereby making Windows software (for an Intel chip) incompatible with Macintosh software (for a Motorola chip).

Output devices:

• A place where instructions and data are held while being manipulated by the processor.
  
• Memory is passive and has no logic circuitry.
  
• There are two categories of memory:

1. Primary Memory: RAM and ROM
   
2. Secondary (or auxiliary) Memory: Storage Devices

• RAM and ROM are chips mounted on the main circuit board along with the CPU.
• Secondary memory devices are mounted elsewhere in the system unit, or external to the system unit.

RAM (Random Access Memory):

• The primary memory that holds data and instructions while the computer is in use.
• Can be thought of as the memory the computer uses to obey instructions.
• Can be written to and read from.
• RAM is "volatile." Its data is lost if the power is turned off.
• If you do not save the information in RAM to a storage device (such as a floppy disk or hard drive) before turning off the computer the information will be lost.

ROM (Read Only Memory):

• ROM is built in memory that permanently stores the initial core operating instructions that enable the computer to "boot."
• The instructions and data in ROM are created when it is manufactured and it cannot be changed.

Secondary (Auxiliary) Memory - Storage Devices:

Secondary memory is "non-volatile" and cheaper than RAM but relatively slow by comparison. It serves as a persistent storage medium. 

• Hard Drive
• Compact Disk/CD
• Flash Drive

Hard Drive:

• is a rigid "hard" metal disk that rotates at high speed. Data is stored by magnetically encoding it on the surface of the disk.
  
• Stores user data in an organized file hierarchy.
  
• Also, stores the computer's operating system and other installed programs.
• Hard drives installed inside a computer are "non-removable," the other media described below are removable.
• "Portable" hard drives have their own case and can be transported among computers and connect via a cable.
• The capacity of a hard drive is usually expressed in terms of megabytes or gigabytes.

Floppy Disk:

• Sony stopped manufacturing in 2010.
• A flexible "floppy" mylar-like magnetic disk mounted in a plastic housing.
• Floppy disks are a "removable" storage medium that allows information to be transported from one computer to another.
• Floppy disks have limited storage capacity, generally 1.44 MB.
• Saving and retrieving information from a floppy disk is slower than on a hard drive.
• They are more susceptible to physical damage than the hard drive.

CD ROM:

• CD ROMs are a high capacity read only storage medium.
• Based on an optical storage technology that uses a low power laser beam to read tiny plastic bumps.
  
• Typically, a CD ROM holds up to 650 MB of information.
• While information retrieval is faster than from a floppy disk, it is still not as fast as from the hard drive.

• A CD-R is highly effective for storing a large amount of data.
• CD-R can hold up to 700MB of information.
• A CD-R is a one time recordable compact disc.

CD-RW:

• A CD-RW allows you to read, write, erase and write again.
• Writing takes place in a single pass of the focused laser beam.
• This is sometimes referred to as direct overwriting and can be repeated several thousand times per disc.

Flash Drive:

• A small, lightweight, removable data storage device that plugs into a computer's USB port.
  
• Contains a rewritable memory chip that, unlike RAM, holds its content without the need of a power supply.
• also known as "pen drives", "thumb drives", or "USB keys."

Memory Capacities - Bits, Bytes, Kilobytes, Megabytes, Gigabytes:

It is important to know that the terms megabyte (MB) and gigabyte (GB) are used when referring to a computer's memory capacity. The number of megabytes or gigabytes reflects the amount of electronic holding places available for instructions or data.

• The processor only recognizes two digits: the "0" and the "1".
• All information is stored as 0's and 1's.
• The 0's and 1's are referred to as bits. (The smallest unit of information on a computer.)
• The next unit of measure is a byte which is a combination of 8 bits. A byte can store one alphabetic character or numeric digit.
  
• A kilobyte (KB) is 1,024 bytes.
• A megabyte (MB) is 1,048,576 bytes.  Equivalent to roughly 500 typed pages of text.
  
• A gigabyte (GB) is 1,073,741,824 bytes.
• Typical file sizes:  Word Document 10KB,  Digital Photo = 100KB,  MP3 Song = 3MB
  Complete works of Shakespeare or 30 seconds of broadcast-quality video = 5MB

Processor Speed - MHz, GHz:

The main indicator of a processor's performance is its speed. Processor speed is measured in Megahertz (millions of cycles per second).  This is a measure of how fast the cpu clock is sending out synchronization pulses that coordinate all of the processor operations. In  2005, current processors are running in the 1800 to 2200 Megahertz range (or 1.8 - 2.2 Gigahertz).

Software is programs and data.

• A program is an organized list of instructions that causes the computer to autonomously execute a predetermined sequence of data manipulations.

1. Applications Software
   

• Application software includes programs that perform specific tasks (e.g., games, word processors, spreadsheets, databases, web browsers).
• Figuratively speaking, application software sits on top of system software because the computer is unable to run without the operating system and system utilities.

2. System Software

• System software includes the operating system and utilities such as the file explorer that provide the basic underlying services shared by all applications.
  
• The operating system perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers.
• The operating system is unique to a specific microprocessor.
  
• The operating system manages and coordinates the hardware for efficient operation.
• The operating system is analogous to the orchestra conductor who supervises the orchestra but doesn't make any music himself.  The applications programs are what do the "real" work for us.

3. Data

• All data is represented in memory as binary numbers - ones and zeroes.
  
• Data is interpreted in a certain way by an application program to represent numbers, letters, images, notes in a song, etc. 
  
• There is nothing about the data itself that tells whether it is an image or a number.
• Data is stored in files so it can be named and retrieved as a coherent unit.
• There are naming conventions for files to indicate the kind of data they contain, but these are just conventions.  
  
• TXT - plain text
• DOC - Microsoft Word document
• MP3- compressed music formt
• JPG - image format
• What would happen if you take an image file, say PORTRAIT.JPG and change the file name to PORTRAIT.DOC?  Does that make it a word processor document? What happens if you try to open PORTRAIT.DOC in Microsoft Word?

HowStuffWorks:
http://computer.howstuffworks.com/hardware-channel.htm
Provides information and pictures on how computers work.