This article introduces the subject of quasi-local horizons at a level
suitable for physics graduate students who have taken a first course on general
relativity. It reviews properties of trapped surfaces and trapped regions in
some simple examples, general properties of trapped surfaces including their
stability properties, the definitions and some applications of dynamical-,
trapping-, and isolated-horizons.Comment: 41 pages, 12 Figures. To appear in the Springer Handbook of
Spacetime, Springer Verlag (2013
