can anyone advice what does speedlite used for how do i know which to buy?

[Surfwooder]

Your 70D is very similar to all the Canon cropped sensor cameras, and does have a pop up flash. The pop up can be used to trigger any flash which uses the flash from the camera to fire. I do know this to be true with the Yn 565 EX. Also, if you mount the Yn 565 to the hotshoe of your 70D you can set it up for E-TTL shooting, by using the flash control in the cameras menu. If you desire to use a radio trigger system, look at the YN 622C transmitter/receiver system. You'll have to buy two 622's since they can be used either as transmitter or receiver, but will transmit and receive E-TTL signals from the camera. E-TTL is Canon speak for "through the lens" metering for flash. The camera will adjust the amount of light from the flash automatically using your in camera metering system. If you want to use this option be sure to buy trigger that supports E-TTL, and not just manual. Good luck.