Ground is essentially just 0 volts. Physically it is the earth and anything directly connected to it so it has the same electric potential (Voltage). For your power supply, all the BLACK wires are ground, as well as the metal casing.
+5V implies an electrical potential of 5V. On your power supply the RED wires are +5V.
Current flows according to the law: Current = Voltage/Resistance where Current is in Amps, Voltage in Volts, and Resistance in Ohms.
This would mean that a 1 Ohm resistor connected to the +5V line, and to the ground, would have 5 amps of current going through it.
Power = Voltage*Current, if Voltage is in Volts, and Current is in amps, Power will be in Watts (joules/second)
And there is your little electrical lesson for the day
If you want to learn more, there is lots on the net and in text books. Look for things like "Ohms law" or "kirchoff's law" to start with. It's all pretty easy stuff, and really usefull to know when you start playing around inside your computer.