Multi User, Multi In, Multi Out
Peeling back MIMO's core functionality revels some pretty nifty techniques on how this is achieved. Spatial Multiplexing is one such technique. Multiple streams are transmitted from different transmit antenna in the same frequency channel. These streams are then re-stitched back together at the receiver.
MU-MIMO advances MIMO techniques further by allowing, say a wireless access point, to simultaneously transmit to multiple clients by intelligently spreading transmit data across all spatial- streams. This allows client devices to take a decent cut of the access points transmit capacity more efficiently.
802.11ac's Wave 2 is just around the corner and optionally introduces wider channels: 160 MHz or two non-contiguous 80 MHz channels or both, effectively doubling the throughput again. Additional spatial streams (i.e. 4) may also be supported on Wave 2 devices.