A typical personal computer consists of a case or chassis in a tower shape (desktop) and the following parts:
Internals of typical personal computer
Inside a Custom Computer
Motherboard
Motherboard - the "body" or mainframe of the computer, through which all other components interface. 
Central processing unit (CPU) - Performs most of the calculations which enable a computer to function, sometimes referred to as the "brain" of the computer.
Computer fan - Used to lower the temperature of the computer; a fan is almost always attached to the CPU, and the computer case will generally have several fans to maintain a constant airflow. Liquid cooling can also be used to cool a computer, though it focuses more on individual parts rather than the overall temperature inside the chassis.
Random Access Memory (RAM) - Fast-access memory that is cleared when the computer is powered-down. RAM attaches directly to the motherboard, and is used to store programs that are currently running.
Firmware is loaded from the Read only memory ROM run from the Basic Input-Output System (BIOS) or in newer systems Extensible Firmware Interface (EFI) compliant
Internal Buses - Connections to various internal components.
PCI
PCI-E
USB
HyperTransport
CSI (expected in 2008)
AGP (being phased out)
VLB (outdated)
External Bus Controllers - used to connect to external peripherals, such as printers and input devices. These ports may also be based upon expansion cards, attached to the internal buses.
parallel port (outdated)
serial port (outdated)
USB
firewire
SCSI (On Servers and older machines)
PS/2 (For mice and keyboards, being phased out and replaced by USB.)
ISA (outdated)
EISA (outdated)
MCA (outdated)
Power supply
Main article: Computer power supply
A case that holds a transformer, voltage control, and (usually) a cooling fan, and supplies power to run the rest of the computer, the most common types of power supplies are AT and BabyAT (old) but the standard for PC's actually are ATX and micro ATX. smps( Switch Mode Power Supply )
Storage controllers
Controllers for hard disk, CD-ROM and other drives like internal Zip and Jaz conventionally for a PC are IDE/ATA; the controllers sit directly on the motherboard (on-board) or on expansion cards, such as a Disk array controller. IDE is usually integrated, unlike SCSI which is found in most servers. The floppy drive interface is a legacy MFM interface which is now slowly disappearing. All these interfaces are gradually being phased out to be replaced by SATA and SAS.
Video display controller
Main article: Graphics card
Produces the output for the visual display unit. This will either be built into the motherboard or attached in its own separate slot (PCI, PCI-E, PCI-E 2.0, or AGP), in the form of a Graphics Card.
Removable media devices
Main article: Computer storage
CD - the most common type of removable media, inexpensive but has a short life-span.
CD-ROM Drive - a device used for reading data from a CD.
CD Writer - a device used for both reading and writing data to and from a CD.
DVD - a popular type of removable media that is the same dimensions as a CD but stores up to 6 times as much information. It is the most common way of transferring digital video.
DVD-ROM Drive - a device used for reading data from a DVD.
DVD Writer - a device used for both reading and writing data to and from a DVD.
DVD-RAM Drive - a device used for rapid writing and reading of data from a special type of DVD.
Blu-ray - a high-density optical disc format for the storage of digital information, including high-definition video.
BD-ROM Drive - a device used for reading data from a Blu-ray disc.
BD Writer - a device used for both reading and writing data to and from a Blu-ray disc.
HD DVD - a high-density optical disc format and successor to the standard DVD. It was a discontinued competitor to the Blu-ray format.
Floppy disk - an outdated storage device consisting of a thin disk of a flexible magnetic storage medium.
Zip drive - an outdated medium-capacity removable disk storage system, first introduced by Iomega in 1994.
USB flash drive - a flash memory data storage device integrated with a USB interface, typically small, lightweight, removable and rewritable.
Tape drive - a device that reads and writes data on a magnetic tape, usually used for long term storage.
Internal storage
Hardware that keeps data inside the computer for later use and remains persistent even when the computer has no power.
Hard disk - for medium-term storage of data.
Solid-state drive - a device similar to hard disk, but containing no moving parts.
Disk array controller - a device to manage several hard disks, to achieve performance or reliability improvement.
Sound card
Main article: Sound card
Enables the computer to output sound to audio devices, as well as accept input from a microphone. Most modern computers have sound cards built-in to the motherboard, though it is common for a user to install a separate sound card as an upgrade.
Networking
Main article: Computer networks
Connects the computer to the Internet and/or other computers.
Modem - for dial-up connections
Network card - for DSL/Cable internet, and/or connecting to other computers.
Direct Cable Connection - Use of a null modem, connecting two computers together using their serial ports or a Laplink Cable, connecting two computers together with their parallel ports.
dial up connections broad band connections
Other peripherals
Main article: Peripheral
In addition, hardware devices can include external components of a computer system. The following are either standard or very common.
Wheel mouse
Includes various input and output devices, usually external to the computer system
Input
Main article: Input
Text input devices
Keyboard - a device, to input text and characters by depressing buttons (referred to as keys), similar to a typewriter. The most common English-language key layout is the QWERTY layout.
Pointing devices
Mouse - a pointing device that detects two dimensional motion relative to its supporting surface.
Trackball - a pointing device consisting of an exposed portruding ball housed in a socket that detects rotation about two axes.
Xbox 360 Controller - A controller used for Xbox 360, Which with the use of the application Switchblade(tm), can be used as an additional pointing device with the left or right thumbstick.
Gaming devices
Joystick - a general control device that consists of a handheld stick that pivots around one end, to detect angles in two or three dimensions.
Gamepad - a general game controller held in the hand that relies on the digits (especially thumbs) to provide input.
Game controller - a specific type of controller specialized for certain gaming purposes.
Image, Video input devices
Image scanner - a device that provides input by analyzing images, printed text, handwriting, or an object.
Webcam - a low resolution video camera used to provide visual input that can be easily transferred over the internet.
Audio input devices
Microphone - an acoustic sensor that provides input by converting sound into an electrical signals
Output
Main article: Output
Image, Video output devices
Printer - a peripheral device that produces a hard (usually paper) copy of a document.
Monitor - device that displays a video signal, similar to a television, to provide the user with information and an interface with which to interact.
Audio output devices
Speakers - a device that converts analog audio signals into the equivalent air vibrations in order to make audible sound.
Headset - a device similar in functionality to computer speakers used mainly to not disturb others nearby.
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