Here is an explanation of QAM from Radio-Electronics.com:
"Quadrature Amplitude Modulation or QAM is a form of modulation which is widely used for modulating data signals onto a carrier used for radio communications. It is widely used because it offers advantages over other forms of data modulation such as PSK, although many forms of data modulation operate alongside each other.
Quadrature Amplitude Modulation, QAM is a signal in which two carriers shifted in phase by 90 degrees are modulated and the resultant output consists of both amplitude and phase variations. In view of the fact that both amplitude and phase variations are present it may also be considered as a mixture of amplitude and phase modulation.
"IEEE 802.11ac is a wireless computer networking standard of 802.11, developed in the IEEE Standards Association process, providing high-throughput wireless local area networks on the 5 GHz band. The standard was developed from 2011 through 2013, with final 802.11 Working Group approval and publication scheduled for early 2014. According to a study, devices with the 802.11ac specification are expected to be common by 2015 with an estimated one billion spread around the world.
This specification has expected multi-station WLAN throughput of at least 1 gigabit per second and a single link throughput of at least 500 megabits per second (500 Mbit/s). This is accomplished by extending the air interface concepts embraced by 802.11n: wider RF bandwidth (up to 160 MHz), more MIMO spatial streams (up to 8), multi-user MIMO, and high-density modulation (up to 256 QAM)."
Yes, there is backwards compatibility, so you should be fine with older devices. There might be some differences noticed in range. Depending on the number of devices using the same access point, you could actually see a decreased range.