There is a thread on here about it, but to summarize:
82 and 83 had STs, which was the lowest trim. STs had no passenger's side mirror (IDK if the driver's side one was power or manual), and had a carb instead of EFI in 83. They may have also only been available with a manual, don't remember. Power steering was either optional or not available. They were otherwise a low spec GT.
GT had a solid rear axle, no flares, drum rear brakes, etc. Power windows were optional on all models, and power mirrors were standard on GT/GT-S.
82 GT-S also had solid rear axle and drum brakes, 83 had IRS and drums, 84/85 had IRS and discs (90% sure that's all right). 85 manuals came with 7.5" F-code diffs, all others used 6.7" T-code. GT-S interior was nicer and similar to the MKII Supra, and they got better seats.
Note: In 82 & 83, there was a "GT Sport" option package, which became the GT-S in 84, but we usually call them all GT-S so we don't go insane.