  In this paper we review the AdS/BCFT proposal of T. Takayanagi for
holographic description of systems with boundaries, in particular, boundary
conformal field theories (BCFTs). Motivated by better understanding of the
proposed duality we employ entanglement entropy as a probe of familiar
properties of impurities and defects. Using the dual gravity description, we
check that in two spacetime dimensions the impurity entropy does not depend on
a particular state of the theory, which is a well-known CFT result. In three
dimensions different, and not necessarily equivalent, definitions of the defect
entropy can be given. We compute the entanglement entropy of a line defect at
finite temperature and compare it with earlier calculations of the
thermodynamical entropy. The results indicate that the entanglement entropy
flows to the definition of the entropy as the Bekenstein-Hawking entropy
associated to a portion of the black horizon, which we call impurity "shadow".
Geometric configurations, which we discuss, provide examples of RG flows of the
defect entropies. We outline the connection between the geometric picture of
the RG flows and examples of lattice calculations. We also discuss some new
generalizations of the AdS/BCFT geometries.
