Volts, Amps, Ohms

The three most basic units in electricity are voltage (V), current (I) and resistance (r).

Definitions:

V = Voltage (Volts)

I = Current (Amps)

R = Resistance (Ohms)

P = Power (Watts)

Analogy:

Plumbing pipes.

voltage <-> water pressure
increase the pressure in the tank -> more water come out of the hose.
increase the voltage -> more current flow.

resistance <-> pipe size.
increase the diameter of the hose -> more water come out of the hose.
decrease the resistance in an electrical system -> which increases the current flow.

In either cases you increase the water flow rate or current flow

Electrical power is measured in watts.
P = VI
Take a hose and point it at a waterwheel like the ones that were used to turn grinding stones in watermills. You can increase the power generated by the waterwheel in two ways.

1) increase the pressure of the water coming out of the hose, it hits the waterwheel with a lot
more force and the wheel turns faster, generating more power.

2) If you increase the flow rate, the waterwheel turns faster because of the weight of the extra water hitting it.

Ohms Law

V=IR

I=V/R

R=V/I

Power

P=VI

Assuming DC

AC calculations introduce something called the "power factor"or Phase shift between Voltages and Currents.

source:
http://www.idc-online.com/technical_references/pdfs/electrical_engineering/Volts_Amps_and_Ohms.pdf