According to the provided details, the new HDMI v2.1 specification will be backward compatible with earlier versions and in addition to higher video resolution and refresh rates, including 8K@60Hz and 4K@120Hz, it will also bring support for Dynamic HDR, Game Mode variable refresh rate (VRR), eARC support for advanced audio formats and the new 48G cable that will provide 48Gbps of bandwidth which is a key for 8K HDR support.
The full list of resolutions and refresh rates start with 4K at 50/60Hz and 100/120Hz and climbs all the way up to 10K resolution at both 50/60Hz and 100/120Hz.
The big surprise is the new Game Mode VRR, which is similar with AMD FreeSync and Nvidia G-Sync, and meant to provide stutter-, tearing- and lag-free gaming, on both consoles and the PC.
Another big novelty is the all new 48G cable, which will provide enough bandwidth for higher resolutions with HDR. The old cables will be compatible with some of the new features, but for 8K/10K with HDR, you will need the new HDMI v2.1 cable.
According to HDMI Forum, the new HDMI v2.1 specification will be available to all HDMI 2.0 Adopters which will be notified when it is released in early Q2 2017.