  We compile and analyze ~200 trigonometric parallaxes and proper motions of
molecular masers associated with very young high-mass stars. These measurements
strongly suggest that the Milky Way is a four-arm spiral. Fitting log-periodic
spirals to the locations of the masers, allows us to significantly expand our
view of the structure of the Milky Way. We present an updated model for its
spiral structure and incorporate it into our previously published
parallax-based distance-estimation program for sources associated with spiral
arms. Modeling the three-dimensional space motions yields estimates of the
distance to the Galactic center, Ro = 8.15 +/- 0.15 kpc, the circular rotation
speed at the Sun's position, To = 236 +/- 7 km/s, and the nature of the
rotation curve. Our data strongly constrain the full circular velocity of the
Sun, To + Vsun = 247 +/- 4 km/s, and its angular velocity, (To + Vsun)/Ro =
30.32 +/- 0.27 km/s/kpc. Transforming the measured space motions to a
Galactocentric frame which rotates with the Galaxy, we find non-circular
velocity components typically about 10 km/s. However, near the Galactic bar and
in a portion of the Perseus arm, we find significantly larger non-circular
motions. Young high-mass stars within 7 kpc of the Galactic center have a scale
height of only 19 pc and, thus, are well suited to define the Galactic plane.
We find that the orientation of the plane is consistent with the IAU-defined
plane to within +/-0.1 deg., and that the Sun is offset toward the north
Galactic pole by Zsun = 5.5 +/- 5.8 pc. Accounting for this offset places the
central supermassive black hole, Sgr A*, in the midplane of the Galaxy. Using
our improved Galactic parameters, we predict the Hulse-Taylor binary pulsar to
be at a distance of 6.54 +/- 0.24 kpc, assuming its orbital decay from
gravitational radiation follows general relativity.
