I think i'm going to be a pain. I'm totally new to this whole thing. I have a basic understanding of electronics and the individual components, but that's where it ends. i have a project in mind and have a basic idea of what i need, i think. so i'm going to ask a bunch of really stupid questions until i do understand. be gentle with me, it's my first time. i would like to build a device that will transmit a simple signal to another, wirelessly to simply tell the second one open or close a relay (triac). i think i understand the roll that the arduino plays, but i'm not sure about the rf. do i need to use something like the xbee shield or can i get away with just a transceiver pair? what i don't know is what kind if signal pattern i need to send and whether or not the arduino can generate that pattern. or is it simply a matter of turning the transmitter on and off. is the carrier signal enough to do what i want? i hope you understand what i'm asking.
I think you need to read a copy of Tom Igoe's book "Making Things Talk"
This answers all the questions you have here and goes through the pros and cons of each possible solution.
You can do it with RF. You can use a simple 4 bit holtek encoder / decoder pair and take outputs from decoder, feed it to a uln2003 and from the uln2003 drive the relay. you can activate the encoder using just 2 pins - 1 for setting 1 bit high or low and 1 for clocking the encoder(giving it a single pulse).
I know its not the best of explanation but then i'm just a bit ahead of you i suppose. I started in the same spot a year back (and i didn't know a triac back then) 
cheers,
Pracas