Beretta like other manufacturers likes to have a myriad of models to confuse buyers and make last years gun obsolete so you will buy the new model.
Basically the 686 series is the base model, usually targeted at hunters.
The 687 models are slightly better in features, having parts that wear normally in heavy shooting easily replaceable unlike the 686.
The 682 series is targeted at serious Target shooters and usually has higher grade wood, engaving etc. Generally built to take more shooting than the lower grades.
The Trident is Berettas attempt to compete with high end guns such as Krieghoff, Perazzi and Kolar. $$$$$$$
The SO series are hand made sidelocks made for the filthy rich, us mere mortals can only drool. Fancy, complicated and not worth the money IMHO.