  This PhD dissertation is devoted to a non-perturbative study of QCD
correlators. The main tool that we use is lattice QCD. We concentrated our
efforts on the study of the main correlators of the pure Yang - Mills theory in
the Landau gauge, namely the ghost and the gluon propagators. We are
particularly interested in determining the $\Lqcd$ parameter. It is extracted
by means of perturbative predictions available up to NNNLO. The related topic
is the influence of non-perturbative effects that show up as appearance of
power-corrections to the low-momentum behaviour of the Green functions. A new
method of removing these power corrections allows a better estimate of $\Lqcd$.
Our result is $\Lambda^{n_f=0}_{\ms} = 269(5)^{+12}_{-9}$ MeV. Another question
that we address is the infrared behaviour of Green functions, at momenta of
order and below $\Lqcd$. At low energy the momentum dependence of the
propagators changes considerably, and this is probably related to confinement.
The lattice approach allows to check the predictions of analytical methods
because it gives access to non-perturbative correlators. According to our
analysis the gluon propagator is finite and non-zero at vanishing momentum, and
the power-law behaviour of the ghost propagator is the same as in the free
case.
