A comparator is relatively easy to figure out. It has two inputs, marked plus (+) and minus (-). One voltage is fed into the "plus" side. Another voltage is fed into the "minus" side. The comparator takes the two voltages and compares them. It issues an output based upon its findings. The output is either on or off. A voltage is either present at the output, or it is not, making it similar to an on-off switch. If the output is on, it is typically referred to as being "high." If the output is off, its referred to as being "low" in electronics jargon.
Some comparators can be programmed for different voltage levels. Suppose you only want the output to go high, only when a difference of 2 volts is present at the plus and minus side. By wiring in ancillary components, you can set the output levels by an adjustable control. The National Semiconductor LM111/LM211/LM311 series of comparators are programmable by wiring various configurations of circuitry to the pins. You can even compare a voltage to ground, by wiring in the minus side directly to ground.
Comparators have thousands of uses, depending how they are wired to other components. One possible use is as a metal detector. A magnetic pickup is wired into the plus and minus side. When a metal is brought near the pickup, the output goes high. Shoplifting alarms at many stores use this principle. When the magnetic anti-theft device is sensed, the output goes high and triggers an alarm.
Comparators can be stacked one behind another. The output of one comparator is fed into the plus or minus input of another comparator. By doing this, you compare a compared voltage. For example; you may want a high output in the second comparator only when the first comparator signals a low output. You may want the second comparator to hold the output until the first comparator's input changes. This "clamp and hold" circuit does not turn off until until some voltage comparison changes in the first comparator. You can even stack three or more comparators, sensing many voltage inputs, and issuing outputs according to your program.