This is a very common question. First, we must warn of the difference between theoretical speed and actual speed. You have probably heard this a million times, but there are so many factors that can affect wireless throughput. I won't go into them here so I can stay on the subject.
802.11n is a three-spatial-stream design that has a theoretical maximum data rate of 450Mbps.
802.11ac is really the next step successor to the current 802.11n standard at frequencies under 6GHz. 802.11ac can operate at 802.11n's 2.4GHz and 5Ghz bands although high speed at 2.4GHz is not anticipated. We have heard some first generation four-stream 802.11ac products that use better coding and error correction to double that rate over 40MHz-wide channels theoretically capable of 867Mbps. Later 802.11ac products are expected to use wider 80MHz and 160MHz channels to double, even quadruple data rates and it is likely that manufacturers will also raise the number of spatial streams to push the speeds as high as possible.
The 802.11ad standard is still in the works. It uses the same technologies as 802.11ac however it uses a 60Ghz band. The much higher frequency limits distance but has a much higher theoretical throughput starting at around 7Gbps. The primary usage will be for docking between devices in a room eliminating things like video and data cables. Think of 802.11ad a a multi-gigabit speed technology. It is being called "WiGig".
I hope this answers the question and I welcome comments.