The PSU

The PSU or Power Supply Unit is what gives your computer power. There are two types or PSU, Modular and non-modular. Modular PSU have detachable cables so you only plug in cables that you need. Non-modular PSU have fixed cables that can’t be removed so you have to have all of the cables. There’re several cables going to the motherboard, these are the following: A small 4 or 8 pin one that goes to the top left of the motherboard, A large 24 Pin connector that goes onto the right hand side of the motherboard and the rest go to video cards and hard drives and optical drives. The suggested power is 350-750W however; you may needs more depending on your configuration. The more power you have, the more things you can add to your PC. Lets say the CPU requires 105W if you have a 500w PSU and the CPU is under full load, the PSU will be able to provide the power so it will be able to run.        

modular

http://www.google.co.uk/imgres?num=10&hl=en&safe=off&rls=ig&biw=1013&bih=1279&tbm=isch&tbnid=2HDJtVPMvbv-sM:&imgrefurl=http://caltechcomputer.com/index.php%3Fmain_page%3Dproduct_info%26products_id%3D454&docid=LlpUyz0KieXPfM&imgurl=http://caltechcomputer.com/images/ps-coolermaster-silentpro850_01.jpg&w=590&h=442&ei=w7GbUNt5hYrQBZKbgMAH&zoom=1&iact=hc&vpx=274&vpy=306&dur=2583&hovh=194&hovw=259&tx=165&ty=110&sig=114947253328420006389&page=1&tbnh=138&tbnw=170&start=0&ndsp=39&ved=1t:429,r:6,s:0,i:87

non modular:

http://www.google.co.uk/imgres?num=10&hl=en&safe=off&rls=ig&biw=1013&bih=1279&tbm=isch&tbnid=XqdNT3Jlzx2vkM:&imgrefurl=http://www.tomshardware.com/forum/346945-31-advice-case-1100-build&docid=Pd6_MxukKQixQM&imgurl=http://images.bit-tech.net/content_images/2008/02/corsair_tx750w_psu_cmpsu-750tx/tx750w3-8.jpg&w=800&h=600&ei=_bGbUNmgLauZ0QWJkIHwDw&zoom=1&iact=hc&vpx=715&vpy=300&dur=305&hovh=194&hovw=259&tx=203&ty=105&sig=114947253328420006389&page=1&tbnh=141&tbnw=188&start=0&ndsp=30&ved=1t:429,r:7,s:0,i:90