Network Card: This is a very necessary item if you want to connect your PC to a local area network (LAN). It all latest motherboard this is an inbuilt module so you don't have to buy it separately. But if you want to configure your PC as a router then you need multiple network cards and in that case you can buy it as a PCI add on card.
Modem: This also comes as an inbuilt module in all latest motherboards. This is useful when you want to connect to internet through a dialup connection. But today everybody is going for broadband connections. Since it is a high speed connection you have to buy a separate broadband modem or router from the ISP like BSNL, Reliance, Airtel etc. If you are planning for an
internet connection via coaxial cable providing by your local cable tv provider then a cable modem is required.
Sound Card: Sound card also embedded into motherboards these days. Normally 3 connections will be there in sound card, Output, Mic and AUX. Sound card is an analog to digital and digital to analog converter an amplifier.
Apart from the above mentioned basic parts here is a list of optional accessories
TV Tuner Card: This is to watch TV programs on your computer screen, costs around Rs1800. TV Tuner comes with a remote and has facilities like recording TV programs, easy channel surfing, save stills, FM radio, set favorite channels etc
Pen Drive: Pen drive is a small memory device which you can connect to any computer through USB port. It is very useful if you want to carry your data with you or can be used to take daily backups. A 512MB Pen drive costs around Rs650 while a 4GB one costs Rs3300. Available brands are Transcend, Sandisk, Kingston, iBall etc
Graphic Card: Now an inbuilt graphic card comes with all motherboard models. So you don't need to buy it separately. But if you are planning your pc for Graphical use like photo editing, animation, graphic designing, play high end games etc then you can go for a high quality graphic card. Many AGP and PCI models are available ranging from Rs1500 to Rs3000.
Scanner: If you want to scan and convert documents or photos to a digital form then buy a scanner. Many models are available from Canon, HP etc
General: So many other accessories and components are available today like iPod, USB MP3 player, wired and wireless headphones, web camera, microphone, external and internal backup devices like tape and magnetic drives, external hard disk/CDROM/DVDROM drives, Gaming devices like joystick, networking devices like hub, switch, router, graphic and text input devices like Optical Mark Recognition, light pen, touchpad, VGA to TV (PAL) converter, digital camera etc