a waste gate is set at a pre determined pressure say "7psi" when air pressure on the intake is at 7psi the waste gate opens by passing the turbo with exhaust gas,this stops the turbo increasing boost as it's missing out on the exhaust energy.
two types of gates internal inside the turbo housing run into your dump pipe and external which can be mounted to feed into your exhaust or dump into a open pipe sometimes called a "scream Pipe"
A blow off valve is set to measure vacume when the throttle close it opens releasing boost pressure to atmosphere,it's purpose is avoid compressor stall during gearchanges as the throttle shuts a pressure wave flows back to the turbo.....its meant to keep to turbo rpm up as it slows under load/backpressure
hope this helps
cheers anthony