  We use a sample of 53 massive early-type strong gravitational lens galaxies
with well-measured redshifts (ranging from z=0.06 to 0.36) and stellar velocity
dispersions (between 175 and 400 km/s) from the Sloan Lens ACS (SLACS) Survey
to derive numerous empirical scaling relations. The ratio between central
stellar velocity dispersion and isothermal lens-model velocity dispersion is
nearly unity within errors. The SLACS lenses define a fundamental plane (FP)
that is consistent with the FP of the general population of early-type
galaxies. We measure the relationship between strong-lensing mass M_lens within
one-half effective radius (R_e/2) and the dimensional mass variable M_dim =
G^-1 sigma_e2^2 R_e/2 to be log_10 [M_lens/10^11 M_Sun] = (1.03 +/- 0.04)
log_10 [M_dim/10^11 M_Sun] + (0.54 +/- 0.02) (where sigma_e2 is the projected
stellar velocity dispersion within R_e/2). The near-unity slope indicates that
the mass-dynamical structure of massive elliptical galaxies is independent of
mass, and that the "tilt" of the SLACS FP is due entirely to variation in total
(luminous plus dark) mass-to-light ratio with mass. Our results imply that
dynamical masses serve as a good proxies for true masses in massive elliptical
galaxies. Regarding the SLACS lenses as a homologous population, we find that
the average enclosed 2D mass profile goes as log_10 [M(<R)/M_dim] = (1.10 +/-
0.09) log_10 [R/R_e] + (0.85 +/- 0.03), consistent with an isothermal (flat
rotation curve) model when de-projected into 3D. This measurement is
inconsistent with the slope of the average projected aperture luminosity
profile at a confidence level greater than 99.9%, implying a minimum
dark-matter fraction of f_DM = 0.38 +/- 0.07 within one effective radius.
(abridged)
