Driving a Sainsmart Relay using Raspberry Pi
This is very old now but I keep getting the link in search results so I thought I would comment in case it helps anyone else reading this as I am. 

In the original diagram it looks like the questioner is thinking that the 2 pins of the relay that his wire is attached to provide current and ground to run a light.  They Don't ...
Its just a switch so you hook the hot wire of a light to one side and hook that same wire to the other side of the relay.  When the relay is powered it switches the switch on and the current can flow thru the relay just as if it were a light switch.
Terrible drawing coming :)

Ground-plug-on-cord------is optional ------------Ground of light/ appliance

Common- plug-on cord ----------------------------Common (usually white) light

Hot wire -----one side of relay---- other end of hot wire to other side of relay

So in my case I did this:
Took my light and carefully cut just the black wire and left the green and white wires connected as they were.  Then I hooked one end of the black wire to one of the terminals on the relay and the other end of the black wire to the other terminal of the same relay.  Then I plugged the power cord into an outlet.
When the relay is closed power flows to the light and it lights (if the light switch is on obviously) when the relay is open no current can flow and ther is no power to the light.

Its not the relay that is supplying power to the light. That comes from the wall outlet.  The relay is just enabling or disabling power from flowing thru the black wire by connecting or disrupting the path for electricity to flow. 

YMMV
Be careful when working with AC power. 

But switching AC power with a SainSmart relay and a raspberry pi is quite straightforward once you understand a little about it.