Power and Ground Clamp Diodes in CMOS IO Buffer
1-2) They are here to protect the device from electrostatic discharge (ESD), by channeling the charge to power rails. ESD can be understood as a very high voltage, very high impedance source. By connecting that source to VCC/Gnd with a diode (very low impedance) the ESD source is depleted from its charge without harming the device.

3) AFAIK, just normal silicon diode

4) Yes, but in a somewhat different version. In LS-TTL there is a (schottky) diode from input to ground, to limit the negative voltage excursions (they can happen depending on input line L/C characteristics). BJTs are not that sensitive to ESD (they have lower input impedance) but they don't like negative voltages above a few volts

These clamping diodes are also useful in that when you need a level translation from high voltage output to low voltage input, you usually can just use a single resistor (in line) that will limit the current, and let the clamping diodes do the voltage translation.