Spacetime is made out of events. An event simply means a moment in time plus a point in space. Events can cause other events, e.g., if a spaceship flies from event A to event B, or a radio signal travels from A to B. It's also possible to have events that can't be causally linked, e.g., if B is 10 light-years away from A and 5 years in A's future, then A can't cause B.
When you have a black hole, an event A close to the center of the black hole can be causally disconnected from any distant event B, even if we place B very far in the future (technically, at null infinity, which is a kind of idealized place, infinitely far away and infinitely far in the future, that a light ray would normally be expected to be able to get to). The black hole has a nearby region which is the set of all events such as A. This region has a boundary, called a black hole event horizon, and a black hole is in fact defined as the kind of object that has this type of horizon. There are other types of event horizons, such as cosmological horizons or the horizon of an accelerated observer.
The event horizon is not an actual physical surface. If you fell through it, you would feel nothing special. Yes, it can basically be interpreted as the point of no return from which light can't escape -- but (a) it's not just light, it's all causal relationships, and (b) the relevant point is that it can't escape to infinity.
WikiPedia says:
The point at which the gravitational pull becomes so great as to make escape impossible.
This is not accurate. It makes it sound as though the event horizon was defined by the strength of a gravitational field, whereas in fact the field can have any strength at the event horizon, depending on the size of the black hole.