The potential discovery of Earth-like planets around other stars will need, apart from a sophisticated technological development, the design of techniques for identifying the most representative parameters of their atmospheres and surfaces. The search for life will constitutes the holy grail of this search. However, for many decades the observations will provide data with low spatial and spectral resolution. In the meantime, the Earth will be for us a sort of Rosetta stone to interpret the exoplanet's spectra. Here we present a review of the main results of simulations and observations looking at our planet with nil or low resolution. In other words, we will study "The Earth seen as a Exoplanet".