What is the difference between AC600, AC1200, etc. 802.11ac wireless network adapters?

[Mark Angelo]

I'm planning to buy myself one soon but I don't understand the difference between different numbered AC network adapters.

 

[Helldog26]

Looks like the AC1200 runs at 1.2Gb transfer speed and the AC600 is only at 433Mbps. The AC1200 essentially is faster.