Diverse physical and astrophysical aspects of black holes are reviewed. We start by describing a membrane paradigm approach in which a black hole is treated as a physical body with very special properties. In particular, a black hole behaves as a conducting sphere with a universal finite electrical resistivity, so that when rotating in an external magnetic field it becomes a unipolar inductor capable of producing a huge potential difference. Astrophysical applications of this mechanism are described and the properties of spacetime inside a black hole are briefly considered. In the bulk of the review, possible sources of observational evidence for the existence of black holes are discussed. Prospects for the detection of gravitational waves from black holes in future by gravitational wave observatories are also examined. The review is concluded with a discussion of the universality phenomenon discovered recently in a study of critical gravitational collapse.