The event horizon is a boundary beyond which information will never reach an observer at a given location in space-time. An event horizon is a mathematical construct and not a physical object and a person passing through an event horizon will not notice any odd behavior. From an outside observer however, an object passing though an event horizon will appear redder and dimmer and will appear to freeze at the moment the object passes the event horizon.
An event horizon can form around a gravitational singularity where its gravitational pull exceeds the ability of any form of mass or energy, including light, to escape. Outside the event horizon, there is a region where light is bent because of the gravitational pull. However it has been demonstrated that a naked singularity where a gravitational singularity exists without an event horizon is possible.
Hypothetically, an event horizon can also exist in a universe, for an observer at a given location in space-time, who remains at the same comoving spatial position. When a universe expands quickly enough, for example a de Sitter universe, it can be possible for an event horizon to exist. 
Other uses of this term: Event Horizon, a science fiction film