Also it's described on any web page that discusses how SPI works. Just typing "SPI bus" in google turns up 11 million results of which the first one (the wiki link) shows how SPI works basically tells us the OP never even attempted a search.
I had a mind to delete this thread when I saw it along with a warning on posting useless threads. Heck the thread tittle is the same as the question text, might as well have only one or the other.