1. It means when you activate (press, flip, or turn) it, it stays in that position after you let go of it.
2. Poles are the number of separate electrical circuits with an identical action a switch or relay has. Your wall switch has One pole, a switch in a stereo may have two or more. Throws are the number of separate positions a switch can be set in. The source knob on a stereo system might have 5 throw and 6 poles. The fan switch switch on a car heater is typically 4 throw.
3. A Relay is basically a switch actuated by a magnetic coil.
4. A relay has a magnetic coil to pull the armature down, and a spring to pull it back up. It depends on the sort of mechanical switch, but generally they have a see-saw which a spring attached to the handle pushes one way or the other, and push switches have a spring which pushes the armature up, and a latch which lets it stay down.
5. NC is the terminal closed to the common terminal when the switch is out or the relay coil inactive, and NO the other way (switch in/coil active).
Common is the common terminal in a pole, which each the throw in a pole connects to ,when the switch is in each respective throw position.
6. Answered elsewhere.