  We show that the gauge invariant treatment of the Schwinger-Dyson equations
of QCD leads to an infrared finite gluon propagator, signaling the dynamical
generation of an effective gluon mass, and a non-enhanced ghost propagator, in
qualitative agreement with recent lattice data. The truncation scheme employed
is based on the synergy between the pinch technique and the background field
method. One of its most powerful features is that the transversality of the
gluon self-energy is manifestly preserved, exactly as dictated by the BRST
symmetry of the theory. We then explain, for the first time in the literature,
how to construct non-perturbatively a renormalization group invariant quantity
out of the conventional gluon propagator. This newly constructed quantity
serves as the natural starting point for defining a non-perturbative effective
charge for QCD, which constitutes, in all respects, the generalization in a
non-Abelian context of the universal QED effective charge. This strong
effective charge displays asymptotic freedom in the ultraviolet, while in the
low-energy regime it freezes at a finite value, giving rise to an infrared
fixed point for QCD. Some possible pitfalls related to the extraction of such
an effective charge from infrared finite gluon propagators, such as those found
on the lattice, are briefly discussed.
