A comparator, as the name suggests, is a device which compares things. It compares voltage levels. On one pin, you connect your reference voltage. The other pin is the test voltage. The output of the chip changes whenever the test voltage is equal to or greater than the reference.
A multivibrator creates an alternating signal, like a 555 timer that generates a square wave pulse. It goes high, low, high, low, high, low... etc and keeps generating that same signal over and over, you just use resistors and a capacitor to determine the frequency that it switches signals.
There are manybi-polar transistors, high gain, high power, switching, but you probably mean why are there NPN and PNP transistors. I suppose the most basic thing would be that if you give a PNP a low input to the base it will source voltage. On the other hand if you give an NPN a high signal, it will provide a sink/ground. You can connect them in ways such that if you give each a common signal which is high, the PNP will turn on and source while the NPN is off. Then if you take the signal low, the PNP turns off, while the NPN will sink.