A Wheststone bridge is basically two voltage dividers in parallel with each other. In the example below, all the resistors are of the same value, so the input voltage across the sensor inputs is zero.
This version is useful when you've got a sensor that has two outputs, one positive and one negative. If you input the sensors' outputs into the two sides of the bridge, then read the voltage across those two sides, you'll get a voltage that's between the bridge's input voltage and ground (+5V in the above schematic). In the photo below, you'll see the sensor attached to a Motorola MPX10 pressure sensor. Thanks to Changsoo Lee for the photo. This PicBasic Pro code will read the sensor when it's wired up in this configuration.