A basic relay is divided into two parts:
1) A magnetic coil (a rectangle with diagonal line on MikMo link drawing), with 2 terminals (1 and 16 on link drawing). This coil works with some voltage. To work with Arduino, you can use 5v. to connect it directly (if coil current comsumption doesn't exceed 40mA - I don't know if exist a relay with less than 40mA of comsumption), or any other voltage, using the example showed by MikMo on link.
You can use a 9v. relay using 9v. source from Arduino Vin, for example, or any other voltage relay with external DC source.
This is the first parameter of relay: [u]coil[/u] voltage and current comsumption. This says what do you need to make coil works.
2) A simple switch with 3 poles:
- 1 central terminal (called common) - P1 on drawing;
- 1 "normally oppened" terminal (called NO) - S1 on drawing;
- 1 "normally closed" terminal (called NC) - O1 on drawing.
These 3 poles can support some voltage and current.
This is the second parameter: [u]Poles[/u] voltage and current limits. This says what you can turn on and off with the relay.
It works like the switch in your room, that turns on the light, but with a contact on the "off" position too. But this switch positions cannot be changed by hand: the coil changes it magnetically for you.
So this switch have 2 positions:
- "Common" short circuit with "NC" when relay coil is not energyzed;
- "Common" short circuit with "NO" when relay coil is energyzed;
So you need a relay with low DC coil voltage (5, 9, 12, 24volts) and with medium AC switch voltage (120 volts in your case) and the switch current depends of your water pump current comsumption (peak value - when motor starts).
I hope this clarify your relay question. It's a very simple component. Don't be frightened! :)