1. A zener diode is a diode with very high doping. Due to this on application of reverse bias, the breakdown happens sharply on increasing the potential beyond a point. Such a breakdown is called Zener Breakdown. In this diode due to very high doping, the diffusion region is very thin, thus for the same potential, the electric field is very high in that region: E= V/L.
thus the breakdown point is reached for a lower potential and is very sharp as compared to normal diodes
2. You can see in the NCERT that the output vs. input graph of the transistor in CE characteristics will be as shown:
Thus we can see that when input voltage is low, output voltage is high and vice-versa. this characteristic of transistor is used to use it as a switch, i.e by controlling input voltage we can switch the output circuit on or off.