Capacitive proximity effect?

Hi guys:

I'm interested in how a capacitive sensor works, in general. I have a vauge idea that the face of the sensor is theoretically like one plate of a capacitor and that a high-frequency signal is applied to it somehow and that an object that comes near the sensing face will change the dielectric and cause a fluctuation of *something*, but I don't know anything more specific than that.

Could someone point me in the direction of a website or book that explains this phenomenon with some explanatory schematics? Or maybe one of you bright fellas can enlighten me.

Thanks for any help you can provide.

Don A. Gilmore Mechanical Engineer Kansas City, Mo.