Wireless networking has had a number of standards which are IEEE 811.x where x has been letters A, B, G, and N. The general public has used the B, G and N standards. In addition to the new AC there is also a new AD standard. 802.11 is the overall wireless standard and usually you just see reference to the letter
A and B were the original standards. B worked at 11 mbps (megabits per second) and was what was originally called Wi-Fi. It worked at 2.4 GHz. (giga Hertz) frequency although you usually see just referenced as 2.4 GHz A was never adopted for popular use but worked at 5 GHz and worked to 54 mbps but did not work with as many devices at a time so was used in industrial needs. G was an upgrade to B and worked at 2.4 GHz and speed was up to 54 mbps. N then appeared and could work with either 2.4 GHz or 5 GHz but was normally set to 2.4 to be compatible with B and G. N can work at speeds from 100 mbps up to about 300 mbps although most devices work in the 100 to 150 mbps range. Currently the term Wi-Fi covers B, G and N.
AC and AD will work on a different frequency than we have been using with B, G, and N. AC and AD will work on the 5 GHz band instead or the 2.4 GHz that B, G and N did. A works on 5 GHz also. N can work at 2.4 (the default) or 5. The GHz is the frequency the wireless is broadcasting at just like an FM station may broadcast at 105.5 and another at 101.3. The 105.5 and 101.3 are similar in way related to how 2.4 and 5 are in wireless.
With AC you will be able to get connection speeds at 1 giga bits per second (gbps) and faster (a bit is a binary digit, 1 or 0, and is the smallest piece of data transmitted, eight bits makes a byte which stores one character we are use) versus where N gave just over 100 (although modifications from the different companies could speed to 300 gbps. Mega is millions and giga is billions so giga bits per second is much faster. AC AD allow double or more of the traffic that N could carry so in some ways it is like we just widened the wireless highway from two lanes to four lanes.
With AD you will be able to go above 1 gigabit per second peed. AD will allow more streams carried at once so will work better for streaming video and also playing games. By using the multiple channels AD will work better with a number of devices attached to it whereas AC will perform best with only a few devices. Which means AC will probably be fine in most homes but AD will be needed in business environments.
If you are going to go to using the higher speeds you will have to upgrade both your wireless access point (also called a router or switch) to have the signal being broadcast at higher sped and you will have to upgrade the wireless network card (NIC) on your computer.
Although IEEE has not made the final approval of the AC standards, it should be approved in early 2014. AD was already approved as a standard. There is equipment appearing on the market using the new standards. I have seen in stores including Target and Walmart, AD and AC equipment from Linksys which is a division of Cisco. Netgear also has equipment available. At this point the prices seem high but it is new and it is a tremendous speed improvement.
AC and AD will not be as backward compatible as G and N were. N devices could work with B and G devices automatically or could be set to work with A devices as N would work on 2.4 GHz or 5 GHz. AC and AD will only work at 5 GHz so it will work with A devices and N devices that are set at 5 GHz. That leaves out B and G.