  Tremendous progress in the science of extrasolar planets has been achieved
since the discovery of a Jupiter orbiting the nearby Sun-like star 51 Pegasi in
1995. Theoretical models have now reached enough maturity to predict the
characteristic properties of these new worlds, mass, radius, atmospheric
signatures, and can be confronted with available observations. We review our
current knowledge of the physical properties of exoplanets, internal structure
and composition, atmospheric signatures, including expected biosignatures for
exo-Earth planets, evolution, and the impact of tidal interaction and stellar
irradiation on these properties for the short-period planets. We discuss the
most recent theoretical achievements in the field and the still pending
questions. We critically analyse the different solutions suggested to explain
abnormally large radii of a significant fraction of transiting exoplanets.
Special attention is devoted to the recently discovered transiting objects in
the overlapping mass range between massive planets and low-mass brown dwarfs,
stressing the ambiguous nature of these bodies, and we discuss the possible
observable diagnostics to identify these two distinct populations. We also
review our present understanding of planet formation and critically examine the
different suggested formation mechanisms. We expect the present review to
provide the basic theoretical background to capture the essential of the
physics of exoplanet formation, structure and evolution, and the related
observable signatures.
