gravity - What's the difference between the apparent horizon and event horizon of a black hole?

The black hole region of a spacetime is defined as a region where nothing can escape to infinity and an event horizon at a given time is the boundary of a connected region of space which is part of the black hole region. As you're after a simple answer I won't give a formal definition of a black hole or an event horizon, but they can be found in Wald.

The problem though is to know whether anything in a region of space can escape to infinity depends on precise knowledge of the future, also not all spacetimes have a suitable notion of infinity. Yet clearly even when we don't know the entire history of the spacetime or when there isn't a suitable notion of infinity, we can still identify objects that are functionally equivalent to black holes. The apparent horizon if you like is the spatial boundary of what we might consider to functionally be a black hole.

As nothing can escape a black hole, even light directed away from it is pulled back towards the singularity so we know that a black hole can cause even light directed outwards, relative to a point in space, to move inwards to that point. The apparent horizon is the boundary between where outwardly directed light moves outwards and where it moves inwards.

In Schwarzschild spacetime, or more generally Kerr-Newman spacetime (in standard coordinates) the apparent horizon and the event horizon coincide. However, more generally, the location of the apparent horizon depends on how you 'slice' spacetime (it is observer-dependent if you like - unlike the event horizon). Apparent horizons needn't be associated with (formally-defined) black holes, however as long as the spacetime has certain properties, they will indeed be associated with black holes and will lie at or inside the event horizon.