I do have a frequent visitor card however how do I get the bar code and what's the difference between the two?

I think one is run by the airport and one is run by HK Immigration.
You need to qualify for the barcode, which you do if you hold a Frequent Traveller Card (there are a couple of other ways too).
When you arrive there is a little booth (there must be a few dotted around the airport) - you go in and have your finger prints taken and your passport registered.
you then get a barcode affixed to the back of your passport.
From now on you can go to an e-channel gate, put your passport in... gate opens... you scan your finger... chit is dispensed giving you x number of days in HK... second gate opens and you're in HK!
Takes 15 seconds. Saves hours over a year compared to normal waiting lines. It is a little quicker than the Freq Trav card line. But it seems the barcode lasts until your passport expires (rather than the card which expires at 2 years).
We think!