The system that is built into your camera/flash uses light pulses/flashes to communicate between the 'master' and the 'slave'. This communication allows you to fire and control the remote 'slave' flash. It gives you automatic flash metering (CLS) etc.
The problem is that light pulses aren't always a reliable communication, especially outdoors. That is why many photographers use radio flash triggers.
Most radio flash triggers only have simple communication, they can only fire the flash. This means that you don't have auto flash metering and you must figure out and set the power manually. It's a bit tricky to wrap your head around but it's fairly easy once you get the hang of it. Actually, a lot of photographers like the manual aspect because it's consistent.
Recently, there are a new products that give us the benefit of radio communication while keeping the auto flash metering and other features of the built-in system. Radio Poppers, for example.
The industry standard product for flash triggering is the Pocket Wizard Plus II. This is a simple radio trigger that will only fire the flash. They are very reliable and their rated range is 1600'. The problem is that they are expensive, something like $180 each, and you need at least two of them. So to answer your question, yes, you would need to buy two, one for the camera and one for the flash. The Plus II is a 'transceiver' so either unit could be a transmitter or a receiver.
Yes, there are cheaper options. The very cheapest ones (that are fairly popular) are the 'Cactus' radio triggers. You can get a set for $40.
Gadget Infinity :: Digital Camera :: Flash Trigger :: Cactus Wireless Flash Trigger Set V4
They aren't known for being too reliable, but it's a great way to test the waters without spending a lot.
There are the Skyports by Elinchrom
Elinchrom EL Skyport Universal Trigger Set-1xTransimtter,1x Receiver Slaves 19360 - Vistek Canada Product Detail
I use the Cyber Sync.
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