When it comes to flashes for modern cameras...there are a couple different ways to go.
Firstly there are 'dedicated' flash units. These will be the new/current models from the manufacturer (Canon)...and a few off-brand models. These flashes can communicate with the camera for metering. The flash fires a pre-flash which is measured by the camera, the camera then tells the flash how much light power will be needed and then the flash fires again. This is called E-TTL metering. This all happens quite fast, you probably won't notice that it fired twice. This means that you don't have to worry about setting the exposure or trying to figure out how to measure the light from the flash. The down side is that these are fairly expensive. I would suggest looking at the Canon 430EX.
You could get a cheaper flash unit, but you probably won't get E-TTL metering.
Some flash units have an auto sensor built into them. The measure the light themselves, but they don't communicate with the camera (only when to fire).
HERE in an example, and
HERE is another. With a flash like that, you would need to match the settings on the flash to the settings on the camera. It's not too hard to do (once you wrap your head around it) but you do have to be aware of it and change with your shooting conditions.
Lastly, you might find a manual flash that doesn't meter at all. It just fires at the power that it's set at (some don't even have power settings...just full power). You can still figure out how to use them because they will have a 'Guide number'. You just divide the guide number by the distance to the subject and that is your F number :er:
