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The two faces of a mechanical watch :
Geartrain side : This is the side you could see if on the back of your watch there was a
glass.
It is from this side is the mechanism that works the watch can be seen.
It is made of the main plate that holds the geartrain, and the bridges.
Terminology
of the geartrain
Functionning:
When you wind up
your watch with the crown, you tighten the barrel spring.
The winding stem  is equipped with a  winding
pinion,
and a sliding pinion (or wig-wag pinion). 
The hole of the
wig-wag pinion is square and slides freely on the square part of the stem.
When you wind up
your watch, the wig-wag pinion is held against the winding pinion through the yoke.
The square part of
the stem drives the wig-wag pinion that in turn drives the winding pinion with its
"Breguet" toothing.
Through to the
crown wheel, the winding pinion drives the ratchet which is fixed on the square part of
the barrel arbor.
The click prevents
the ratchet from coming back, that would unwind the spring barrel.
The barrel arbor
 moves freely in the barrel drum 
The
barrel spring  is fixed to the arbor
by the hook, and to the drum on the large inner diameter .
To sum up :
When you wind up
your watch, you tighten to its maximum the spring around the barrel arbour.
Then, when your
watch works, the spring slowly unwind. As a result, it rotates the barrel drum which in
turn transmits the driving force to the geartrain.
The driving force
reaches the escapement. The escapement transmits gradually the driving force to the
balance.
Thanks to this
force, the balance swings, and your watch works.
Winding Mechanism side
This is the side
of your watch that is under the dial and the hands.
On this side are
the winding mechanism and the hand-setting mechanism.
Winding terminology
Working:
See above for the
reassembling
The hand-setting:
When you pull the winding stem in order to set your watch, the setting lever, thanks to
its stud that enters into the recess of the stem, is set into motion. This move of the
setting lever pushes the yoke forward.
Consequently, the sliding pinion engages with the intermediate wheel.
Then, the intermediate wheel engages with the minute wheel that in turn engages with the
cannon-pinion, that holds the minute hand, and with the hour wheel, that holds the hour
hand.
In the meanwhile,
the winding pinion is free and cannot engage the crown wheel because the sliding pinion
and the winding pinion are no longer in contact with one another.
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