That's a cool ani gif
As with "geoffreym" don't quote me, I am no expert but my understanding was the bushing is the insert you put into the gearbox case. My original ones were in plastic and they reduce the gearbox 'hole' size to match the pin sticking out of the gear. They come in 6, 7, and 8mm sizes depending on the size of the holes in the gearbox (I had to buy 6mm and went with metal to replace the dead plastic which had snapped). You can also get them in oil retaining, ball bearing and other flavours.
Shims are as geoffreym's comments, small discs of metal (washers to me but much thinner) which you put under gears to help sort out spacing.
Tappet plates ARE NOT the part that determines fire-mode. That would be your selector plate.
Tappet plates pull the air nozzle out of the way of the feed. They are attatched to the air nozzle, fit through a slot in the cylinder head, have a spring pulling them forwards and are pulled backwards by a small notch on the sector gear.
Bearings and Bushings are similar, they are what fit into the 6/7/8mm gearbox shell to make the holes a uniform and precise size for the gear shaftf to fit in. Bushings are simpley machined parts, bearings are bushings which spin more freely - in general use bearings, less wear and a higher ROF, use bushings for high stress jobs.
Shims are very thin, very accurate metal washers (they are often quoted in 4/5 sizes of 0.1mm, 0.15mm, 0.2mm and 0.3mm), they are used to space the often poorly fitting components (gears in particular) this means the gears wobble less, are aligned better and basically they make the gears run smoother with no clashing or interference.
well i haven't yet broken a rifle, so i'm giving myself a pat on the back, and even though i don't know the names of the parts, i can still strip and clean one very well.
(that wasn't meant in sarcasm, i learnt the hard way how to do things lol)