bascially treed saddles have a tree, which is a solid frame work, the leather of the saddle is attached around this tree.
A tree in a saddle has to fit a horses back perfectly otherwise it can cause problems like muscle atrophy.
A treeless saddle does not have this solid frame work, but the shape of the saddle is formed by layers of shock absorbing materials, some have solid blocks front and back to create shape.
The stirrup bars are not attached to anything solid, which can cause problems if the correct padding is not used with the saddle, or the rider is too heavy in the stirrups, which they can get away with in a treed saddle.
A treeless saddle is more adjustable than a treed saddle, one saddle can fit a variety of horses and riders, but as with treed saddle the type of saddle needs to be suitable for the horse and rider.
Treeless saddles can be gulleted or gulletless, which means that some have panels underneath that allow a gap for spine and wither clearance, some don't, with the ones that don't the correct padding, which usually creates a gullet for the saddle, is very important.usually the gullet aids the lateral stability of the saddle as well.