This is one of the properties of a sinusoid (sine wave). Phase shift is the amount along the xaxis that the wave's period is shifted. It is quite simple to create an equation for a phase shifted wave, but some background is necessary.
The basic equation for a sine wave is y=sin(x). This means that for a given x value, the y value is the sine of the angle x. For simplicity, we're working in radians (though phase shift can be measured in either radians or degrees), so if we set x to equal, say, π/3, y will equal 1/2.
If we wish to shift this wave along the xaxis, we need to "trick" the sine function that we're feeding it a different x value. Realize that, and it's easy as π.
So, let's say we have the sinusoid represented by the equation y=sin(xπ/2). If an ordinary sine wave(y=sin(x)) looks like this:
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+\/
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 \__/

Then the sinusoid y=sin(xπ/2) makes a graph like this:
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+/\
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 \__/

As you can see, the yvalues have changed as if the x value was π/2 less than what they were (obviously). There is (amazingly), an application for this phenomenon as well, known as Phase Shift Keying. In a nutshell, this uses relative phase shifts as representations of binary values for communication using waveforms.
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