This is kind a technical I guess. Please explain how a GPU actually decodes video in an accelerated manner? Is it very much analogous to video decoding done by CPU? Please explain in steps. Thanks in advance.
You are asking for a detailed description of how CUDA (Nvidia) or STREAM (ATI/AMD) works. I suggest you to those respective sites (nvidia.com and ati.com) and research the various whitepapers that are available.
Why are you asking? This sounds like an academic question.