It's capacitance in nearly all touchpads. Dry skin reduces conductivity. Cold hands become drier, because of decreased capillary blood flow.
Most touchpads are produced with a fine grid beneath the surface, like a sheet of finely ruled graph paper. So they can detect a single, double, triple, or even quadruple set of simultaneous fingertips. Mine (Dell Inspiron) can sense a single finger tip as a left-click, and double fingertips as a right-click. It's capable of sensing "gestures" of up to four fingers, but I leave that feature turned off, since it would provoke too many middle fingers.
The biggest headache I have with my touchpad is that random, unintended contact with the touchpad while I'm in the midst of typing will all too frequently send the cursor elsewhere, resulting in bizarre typos. And the shallow, Hall-effect keys of the keyboard provide far too little tactile feedback. As a codger who learned to type on an ancient typewriter, which required significant finger force, and as much as 1/2 inch of key travel in order to type each letter, my hands have difficulty attending to random, light pressure. I used to cruise at 80 words per minute on those behemoth, steeply angled typewriter keyboards, with hands dancing. The flat electronic keyboards completely change the geometry. On a good day, I can manage maybe 50 wpm on my Dell. And having a "gotcha!" trap immediately below the space bar drives me nuts.
Bob