WSJT is a computer program used for weak-signal radio communication between amateur radio operators. The program was initially written by Joe Taylor, K1JT, but is now open source and is developed by a small team. The digital signal processing techniques in WSJT make it substantially easier for amateur radio operators to employ esoteric propagation modes, such as high speed meteor scatter and moonbounce.
WSJT’s communication modes can be divided into fast and slow modes. While fast modes send character-by-character without error correction, the slow modes aim to optimize for minimal QSO (high-power) use. As of WSJT10, supported fast modes are JTMS, FSK441, ISCAT, and JT6M, and the slow modes are JT65 and JT4. WSJT-X 1.8 additionally implements the “slow” JT9, FT8, and QRA64. Some modes have derived submodes with larger tone spacing. Two other modes, WSPR and Echo are included for measuring propagation and testing moon bounce echo.
From Wikipedia
WSJT-X implements communication protocols or “modes” called FT8, JT4, JT9, JT65, QRA64, ISCAT, MSK144, and WSPR, as well as one called Echo for detecting and measuring your own radio signals reflected from the Moon. These modes were all designed for making reliable, confirmed QSOs under extreme weak-signal conditions. All but ISCAT use nearly identical message structure and “source encoding,” the efficient compression of standard messages used to make minimal QSOs. JT65 and QRA64 were designed for EME (“moonbounce”) on the VHF/UHF bands; JT65 has also proved very popular and effective for worldwide QRP communication at HF. JT9 is optimized for the LF, MF, and HF bands. It is about 2 dB more sensitive than JT65 while using less than 10% of the bandwidth. With either JT9 or JT65, world-wide QSOs are possible with power levels of a few watts and compromise antennas. JT4 and QRA64 are optimized for EME on the VHF and higher bands, and especially the microwave bands from 2.3 to 24 GHz. FT8 is operationally similar to JT65 but is much faster, using T/R cycles only 15 s long. MSK144 is used for Meteor Scatter on the VHF bands. Finally, as described more fully on its own page, WSPR mode implements a protocol designed for probing potential propagation paths with low-power transmissions. WSPR is now fully implemented within WSJT-X, including automatic band-hopping.
From WSJT-X