An accessory is a distinct piece of hardware that is required to use a computer, Essentially, accessories are everything except the ccomputer itself, such as memory, power adapters (AC), and audio/visual cables. The companies that manufacture computers also make these accessories for replacement purposes as well as improving the overall experience . There is an entire industry of companies that create accessories for Pc, called third-party companies and the prices are often lower than those made by the maker of the PC.

Add-on may refer to: a software bundle that allows the user to extend and customize a based applications. Extensions can be used to modify the behavior of existing features to the application or add entirely new features.

Typical PC hardware
A typical personal computer consists of a case or chassis in a tower shape (desktop) and the following parts:

Motherboard - .
Central processing unit (CPU) -
Computer fan
Random Access Memory (RAM)
PCI , PCI-E , ISA , USB , HyperTransport , CSI , AGP , External Bus Controllers .
Power supply
Video display controller
Graphics card
Removable media devices
Main article: Computer storage, CD (compact disc) , CD-ROM Drive , CD Writer,  DVD (digital versatile disc), DVD-ROM Drive ,DVD Writer,  DVD-RAM Drive, Blu-ray . ,BD-ROM Drive , BD Writer , HD DVD  , Floppy disk,  Zip drive . ,  USB flash drive ,  Tape drive .
Internal storage
Hard disk
Sound card
Networking Devices
MPeripheral
Text input devices
Pointing devices
Gaming devices
Image, Video input devices
Image scanner -
Audio input devices
Image, Video output devices
Printer
Monitor
Audio output devices
Speakers
Headset

Other Components & Accessories
Computer Add-Ons ,CPU Processors , Computer Cases ,Computer Speakers , Docking Stations , Drive Enclosures , Drives & Storage , Game Hardware ,Graphics Cards ,I/O Cards , Internet Appliances , KVM Switches , Memory , Mice & Keyboards , Modems , Monitors & Projectors, Motherboards , Networking & Online Communication , PC Components , Presentation Pointers ,Printers , Scanners , Sound Cards , Video Capture & Editing Devices ,Webcams  Headsets , Microphones , All in One Printers, Photo Printers , Inkjet Printers,  Laser Printers ,Scanners Photo , Paper Printer,  Cables,  Ink & Toner ,Flat Panel TVs , DVD Players,  DVD Recorders,  Home Cinema Systems ,  Freeview Adapters,  TV Stands , TV & Audio,  Leads Projectors ,  Apple iPods,  iPod Accessories,  MP3 Players,  Multimedia Players ,  Portable CD,  Stereos Headphones , Radios Hi-Fi , Dictation Machines Speakers ,  TFT Monitors / LCD Monitors ,  cables, Cameras, Cheats, Updates, Dance Mats, Gamepads, Controllers, Control Pads, Graphics Cards, Guns, Headsets, Joysticks, Keyboards, Mice, Memory Cards.  Apple Monitors , Internet Comms , USB,  Gadgets,  PC Speakers,  TV Cards , Web Cams, Mice & Keyboards, USB Expansion ,Graphics Tablets ,  Security , Peripherals Modems,  PC Gaming Accessories ,  Sat Nav Systems, Sat Nav Accessories , Sat Nav Fitness Devices , In-Car DVD , In-Car Audio , DVD-RW Drives , CD and DVD Discs,  Portable data Storage ,

Emachines,  Fujitsu, Siemens,  HP,   Lenovo, Gateway, Dell, Compaq, toshiba,  Advent cybertron, systemax, maxtor, lexmark, hitachi,  epson, IBM , BenQ, creative, cables to go, edge, exabyte, intel, kbyte, kodak,  lexar, nec,  nikon, nvidia, palm, plextor,  sonicwall, trendnet, umax, usrobotics, viewsonic, viking components, vision, wintergreesys, xerox, xfx..,Acer , Antec , Apple, Belkin, Brother , Buffalo , Canon , Case Logic , CH PRODUCTS ,ColorVision,Crucial Technology , D-Link , Ecamm Network ,Elgato, Fuji ,Hauppauge , Hewlett-Packard ,Honeywell,Iogear , Iomega , JBL ,Kensington , Keyspan ,Kingston , Klipsch , LaCie ,Linksys , Logitech , Macally,  Microsoft , Motorola , Neat Receipts , Netgear,Olympus  , Peerless , Plantronics , PNY , Samsung ,
SanDisk , Seagate Secure Digital ,SimpleTech ,Sling Media , Sony , Speck Products ,Startech.com ,
Targus , Think Outside ,Verbatim ,Wacom, Western Digital,

Memory, Optical Drives ,OEM Hard Drives ,OEM Cases, Power Supplies , Processors ,Motherboards, Graphics Cards ,Sound Cards ,Component Cards , Case Fans & Cooling, Internal Cables, Toolkits, Screws and Thermal Paste, Laptop Parts, Performance Parts, SD DIMM  Memory, DDR ,  DDR2 , SD , DDR, DDR2 SODIMM  Memory , Desktop Optical Drives, Laptop Optical Drives,  3.5-inch Hard Drives, 2.5-inch Hard Drives,  Enclosures,  AMD Motherboards,  Intel Motherboards,  nVidia Graphics Cards, ATI Graphics Cards,  Physics Cards,  CPU & GPU Coolers, Case Fans & Coolers.