I bought a CD from an Ebay auction that had PDF files of many Kenwood service manuals, including the one for the TK-790. The CD was a pirated copy and I'm sure Kenwood wouldn't be too happy about their copyright being violated. If you look around on the web, though, you might find some useful files. Check the Kenwood-oriented Yahoo groups, too.
DTMF is an acronym for Dual-Tone Multi-Frequency. DTMF was developed by the Bell System for signalling across telephone lines. When you dial a "touch-tone" phone, you are sending DTMF tones. Some repeaters require a DTMF sequence in order to access the repeater or its special features. And some repeaters allow you to place a phone call through a phone patch attached to the repeater. This is all done by sending DTMF tones.
QT and DQT are Kenwood's nomenclature for CTCSS and DCS, also known by their Motorola tradenames PL and DPL. PL is a continuous low-frequency tone that is mixed with the transmit audio from your radio. If the corresponding receiver is set up to decode PL, it will open its squelch only when the correct PL tone is heard from your transmitter. DPL works the same way except that the code is a very brief digial burst at the beginning of each transmission.
Either the DOS or Windows versions of the TK-x90 programming software has help screens. Press F1 when you have the cursor over a feature you don't understand and the help screen will pop up with a brief explanation.
I have a TK-790H (VHF High, 110 watts) and a TK-890 (UHF, 35 watts). I am currently running the 790 in my mobile and I'm very impressed with its features and capabilities. My plan is to get the dual-band kit so I can run both the 790 and the 890 from one control head.