Question 1:
What is the difference between FBO and Framebuffer ?
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Not sure what you mean, FBO just means framebuffer object, it's the basic opengl offscreen framebuffer, I don't understand in what context you want to know the difference.

Question 2:
Where it will exist either in RAM or VRAM?
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That's up to your driver to decide. Probably vram, though it could be swapped if the driver wants to.

Question 3:
If we are rendering to Texture using FBO,Is it possible to display the contents of FBO just by calling swapbuffers()
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No.

Question 4:
Is any other ways are there to take the FBO (Off Screen Buffer Content) to display buffer other then Framebufferbilt functionality,If so which one is faster ?
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Just use glBlitFramebuffer, that's what it's there for.

Question 5:
Please suggest me the direct way to show the FBO content on to the screen ?
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Same as #4.


http://www.opengl.org/wiki/Framebuffer_Object

If i am using Double buffering,

1) How many FBO (off screen buffer) we can create?
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Very many, probably unlimited, or at least way more than you will need. There's no specific number limit, you'll just be restricted by your graphics hardware and system memory limitations.

2) how openGL know this is the buffer to be display.
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When you have a double buffering setup, OpenGL creates two memory sections, the "front buffer" and the "back buffer". These always exist no matter how many FBOs you create, and are separate from them. When you swap buffers, it always swaps between the back buffer and the front buffer. When you create another framebuffer object (FBO), this defines a new region of memory that is not associated with the front/back buffers. Things you draw to this FBO do not appear in the front/back buffer.

That's why when you're done rendering to your FBO, you have to blit the color information from the FBO to the "back buffer". Because you can't change what surface gets swapped onto the screen, you have to copy whatever you want to appear into the special memory section that is the backbuffer.

3) Is the display buffer is independent from display buffers?
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Don't understand this, but hopefully my above answer has helped you.

Read over the link I posted at the bottom of my first reply, that has some good info. Also the red book describes this all in detail if you're interested in picking up a copy.