Although it sounds disturbingly like a new rig from Yaesu, FT8 is actually a weak signal QSO mode from K1JT, Joe Taylor, and the folks who gave us WSPR and other modes. The mode is Franke-Taylor design, 8-FSK modulation.
I've just had my first contact with Steve VK6IR who is located right across this wide brown land.
I saw his CQ, received at -4dB, double clicked the line and the rest of the QSO, including signal reports each way, was automated.
Unlike WSPR where each over takes two minutes, the cycle time in FT8 is just 15 seconds so it feels a lot more like you're actually having some sort of conversation.
This new mode is in the latest builds of WSJT-X, version 1.8.0-rc1 in my case.
The release notes describe FT8 as follows:
"New mode called FT8: sensitivity down to -20 dB on the AWGN channel; QSOs 4 times faster than JT65 or JT9; auto-sequencing includes an option to respond automatically to first decoded reply to your CQ."
AWGN is Additive white Gaussian noise, obviously.
I kind of wish this was all a little bit more manual and that I could have a few more characters to personalise the QSO. Maybe we need a Twitter like digital mode?