  Multiple images, giant arcs, Einstein cross, fold, cusp, lip, caustics,
critical lines, sources, mapping, time delay, arclets, weak shear,
magnification bias, depletion, ellipticities, polarization, smearing,
convergence, kernel, mass reconstruction, sub-structure, dark halos, natural
telescope, amplification, distant galaxies, faintest sources ... This is a non
exhaustive list of favorite terms used when using gravitational lensing in
clusters of galaxies. I will introduce here lensing in clusters as a useful
tool in modern observational cosmology. I will give a summary of what we are
learning in terms of cluster mass distribution in the strong and weak regime,
and what information we will gain in terms of the cluster physics. I will
underline the benefit of using cluster-lenses as natural telescopes to probe
the distant Universe.
