  We present radio-to-optical data taken by the WEBT, supplemented by VLBA and
RXTE observations, of 3C 279. Our goal is to use this extensive database to
draw inferences regarding the physics of the relativistic jet. We assemble
multifrequency light curves with data from 30 ground-based observatories and
the space-based instruments, along with linear polarization vs. time in the
optical R band. In addition, we present a sequence of 22 images (with
polarization vectors) at 43 GHz at resolution 0.15 milliarcsec, obtained with
the VLBA. We analyse the light curves and polarization, as well as the spectral
energy distributions at different epochs, corresponding to different brightness
states. The IR-optical-UV continuum spectrum of the variable component
corresponds to a power law with a constant slope of -1.6, while in the 2.4-10
keV X-ray band it varies in slope from -1.1 to -1.6. The steepest X-ray
spectrum occurs at a flux minimum. During a decline in flux from maximum in
late 2006, the optical and 43 GHz core polarization vectors rotate by ~300
degrees. The continuum spectrum agrees with steady injection of relativistic
electrons with a power-law energy distribution of slope -3.2 that is steepened
to -4.2 at high energies by radiative losses. The X-ray emission at flux
minimum comes most likely from a new component that starts in an upstream
section of the jet where inverse Compton scattering of seed photons from
outside the jet is important. The rotation of the polarization vector implies
that the jet contains a helical magnetic field that extends ~20 pc past the 43
GHz core.
