  It is hard to imagine curved spacetimes of General Relativity. A simple but
powerful way how to achieve this is visualizing them via embedding diagrams of
both ordinary geometry and optical reference geometry. They facilitate to gain
an intuitive insight into the gravitational field rendered into a curved
spacetime, and to assess the influence of parameters like electric charge and
spin of a black hole, magnetic field or cosmological constant. Optical
reference geometry and related inertial forces and their relationship to
embedding diagrams are particularly useful for investigation of test particles
motion. Embedding diagrams of static and spherically symmetric, or stationary
and axially symmetric black-hole and naked-singularity spacetimes thus present
a useful concept for intuitive understanding of these spacetimes' nature. We
concentrate on general way of embedding into 3-dimensional Euclidean space, and
give a set of illustrative examples.
