Most strain gauges will operate in either posive or negative strain modes, ie pulling or pushing. This changes the resistance of a metal sliver (or comb, to get a bigger response) which is converted into a voltage using a Wheatstone bridge. The bugbear is that the characteristics change with temperature, so temperature compensation is required. I think the way it has been written here is not clear, but temperature is implied for the compensation.

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Note added at 16 mins (2010-07-06 10:43:06 GMT)
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For "comb", read "labyrinth". This effectively makes the metal path much longer, and the resistance (and change in resistance) proportionately greater.