  The DASI discovery of CMB polarization, confirmed by WMAP, has opened a new
chapter in cosmology. Most of the useful information about inflationary
gravitational waves and reionization is on large angular scales where Galactic
foreground contamination is the worst. The goal of the present review is to
provide the state-of-the-art of the CMB polarization from a practical point of
view, connecting real-world data to physical models. We present the physics of
this polarized phenomena and illustrate how it depends of various cosmological
parameters for standard adiabatic models. We also present all observational
constraints to date and discuss how much we have learned about polarized
foregrounds so far from the CMB studies. Finally, we comment on future
prospects for the measurement of CMB polarization.
