What is a GPU?
GPU stands for Graphics Processing Unit. This is the term for the component in your computer that does all the graphics processing, like playing movies, render 3d scenes, play games, but also all the normal things you do like browsing, emailing, etc. require a GPU for the window management. This is the way windows are displayed, how fast menus react upon clicking, window dragging, dropping, and resizing windows.
It is the essential interconnect between the computer and the display. This is why mostly things like coloured stripes, random blocks or any distorted video, are related to GPU or driver problems. Basically, most new IGP’s(integrated graphics processors) are good enough for all normal usage. Only if you want to do fancy things like gaming, 3d modeling or have a LOT of displays connected to your computer, you’ll need a dedicated GPU. For all things like browsing, emailing, textprocessing, programming(depending on what you program) and even watching Full-HD movies, a modern(!) IGP is sufficient. I say modern, because the last few years graphics of desktop environments greatly improved(see the difference between Windows XP and 7), which is cool, but this is also heavier on your GPU. So, modern IGP’s are made for this load, older ones aren’t.
So, you need a powerful GPU is you want to play games on their highest settings or render 3d scenes or things like that. For things like text processing, browsing the web and emailing, you don’t really need a dedicated GPU, so then a built-in or light-weigth GPU would suffice.