Question: What is the effect where clocks near a black hole appear to tick more slowly?
About the article "Black hole", section "Event horizon".

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The defining feature of a black hole is the appearance of an event horizon—a boundary in spacetime through which matter and light can only pass inward towards the mass of the black hole.
Nothing, not even light, can escape from inside the event horizon.
The event horizon is referred to as such because if an event occurs within the boundary, information from that event cannot reach an outside observer, making it impossible to determine if such an event occurred.
As predicted by general relativity, the presence of a mass deforms spacetime in such a way that the paths taken by particles bend towards the mass.
At the event horizon of a black hole, this deformation becomes so strong that there are no paths that lead away from the black hole.
To a distant observer, clocks near a black hole appear to tick more slowly than those further away from the black hole.
Due to this effect, known as gravitational time dilation, an object falling into a black hole appears to slow down as it approaches the event horizon, taking an infinite time to reach it.
At the same time, all processes on this object slow down, for a fixed outside observer, causing emitted light to appear redder and dimmer, an effect known as gravitational redshift.
Eventually, at a point just before it reaches the event horizon, the falling object becomes so dim that it can no longer be seen.
On the other hand, an indestructible observer falling into a black hole does not notice any of these effects as he crosses the event horizon.
According to his own clock, which appears to him to tick normally, he crosses the event horizon after a finite time without noting any singular behaviour.
In particular, he is unable to determine exactly when he crosses it, as it is impossible to determine the location of the event horizon from local observations.
The shape of the event horizon of a black hole is always approximately spherical.
For non-rotating (static) black holes the geometry is precisely spherical, while for rotating black holes the sphere is somewhat oblate.
As Einstein predicted, relativistic effects can also cause clock drift due to time dilation.
This is because there is no fixed universal time, time being relative to the observer.
Special relativity describes how two clocks held by people in different inertial frames (i.e.
moving with respect to each other but not accelerating or decelerating) will each appear to tick more slowly to the other person.
In addition to this, general relativity gives us gravitational time dilation.
Briefly, a clock in a stronger gravitational field (e.g.
closer to a planet) will appear to tick more slowly.
People holding these clocks would agree on which clock appeared to be going faster.
Note that it is time itself rather than the function of the clock which is affected.
Both effects have been experimentally observed.
Time dilation is of practical importance.
For instance, the clocks in GPS satellites experience this effect due to the reduced gravity they experience (making their clocks appear to run more quickly than those on Earth) and must therefore incorporate relativistically corrected calculations when reporting locations to users.
If general relativity were not accounted for, a navigational fix based on the GPS satellites would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day. 
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General relativity predicts an additional effect, in which an increase in gravitational potential due to altitude speeds the clocks up.
That is, clocks at higher altitude tick faster than clocks on Earth's surface.
This effect has been confirmed in many tests of general relativity, such as the Pound–Rebka experiment and Gravity Probe A.
In the Hafele–Keating experiment, there was a slight increase in gravitational potential due to altitude that tended to speed the clocks back up.
Since the aircraft flew at roughly the same altitude in both directions, this effect was approximately the same for the two planes, but nevertheless it caused a difference in comparison to the clocks on the ground. 
: special vs. general theories of relativity.
In Albert Einstein's theories of relativity, time dilation in these two circumstances can be summarized:  In special relativity (or, hypothetically far from all gravitational mass), clocks that are moving with respect to an inertial system of observation are measured to be running more slowly.
This effect is described precisely by the Lorentz transformation.
In general relativity, clocks at lower potentials in a gravitational field – such as in closer proximity to a planet – are found to be running more slowly.
The articles on gravitational time dilation and gravitational red shift give a more detailed discussion.
Special and general relativistic effects can combine (as seen with ISS astronauts).
In special relativity, the time dilation effect is reciprocal: as observed from the point of view of either of two clocks which are in motion with respect to each other, it will be the other clock that is time dilated.
(This presumes that the relative motion of both parties is uniform; that is, they do not accelerate with respect to one another during the course of the observations.)
In contrast, gravitational time dilation (as treated in general relativity) is not reciprocal: an observer at the top of a tower will observe that clocks at ground level tick slower, and observers on the ground will agree about the direction and the ratio of the difference.
There is still some disagreement in a sense, because all the observers believe their own local clocks are correct, but the direction and ratio of gravitational time dilation is agreed by all observers, independent of their altitude.