Baffle trims are commonly used in household settings. They have a ribbed interior where some of the light is absorbed by the trim, softening the overall light entering the room.
Reflector trims, on the other hand, have no ribbing inside the trim. They have smooth and highly polished interiors, helping to push as much light out of the can as possible. These can be useful in commercial settings, and can also be found in homes in the case of high ceilings or any place where light needs to be maximized -- kitchens or other utility areas, for instance.
These terms are useful to know when you're installing trims in a recessed lighting fixture, but they can also come in handy when installing LED downlights into those fixtures, as some downlights offer the option baffle and reflector trim options.