Why Q65 is the newest of the WSJT-X weak-signal modes, built on JT65's 65-tone FSK framework but with a stronger Q-ary repeat-accumulate error-correcting code. Its key operational advantage is message averaging: the software can accumulate and combine several successive receive sequences of the same message, pulling a signal out of the noise that no single 60-second cycle could decode. That makes it the choice for extremely weak paths such as EME, ionospheric scatter, and troposcatter on VHF/UHF, as well as very weak HF work.
Watch out The quadrature modulation answer misreads the name: the Q refers to the Q-ary (65-symbol) code, not QPSK, since the mode is still multi-tone FSK. Keyboard-to-keyboard chat is not supported either, because Q65 uses structured, fixed-format messages like the other WSJT-X modes.
Q65: Q is for Q-ary code, and its trick is stacking multiple receive cycles (averaging) to decode.