What exactly has to happen for a stocks value to go up or down?

I would appreciate it if one or more of the smart people on this blog can explain the following : what causes individual stocks prices to change - be it higher or lower ? For every seller there has to be a buyer, so what exactly is causing a stock price to move up or down? What determines how much that movement will be ? Is there a mechanism that continuously, and at millisecond or nanosecond intervals, keeps track of the number of sell orders waiting in a queue for a buyer ? If so, how does this lead to a .25% gain or loss in a stocks value vs a 0.5 % gain or loss ? Twice as much of "what" has to take place to go up or down 0.25% vs 0.5 %. Is it something else ?

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