For our purposes occultations are events where one celestial object (partly)  covers another as seen by an observer from Earth.
Objects which tend to occult others are:

• Sun
• Moon
• Planets
• Planets moons
• Asteroids
• Comets

The occulted objects can be any of the fore mentioned and, of course, the stars. And many of these occultations are among the most wonderful phenomena to observe! Think for instance of:

• Solar eclipses (the Moon occults the Sun)
• Lunar eclipses (the Moon passes through the Earth's shadow)
• The Moons of Jupiter or Saturn occulting one another
• Asteroids passing in front of stars (they briefly appear 'turned off')
• The Moon occulting all of the other possible objects.

When the Moon passes in front of e.g. a star we observe a disappearance. After a while the star while reappear on the other side. Because the Moon has no atmosphere  (worth mentioning)  and the sizes of stars as seen from Earth are so extremely small, we see stars most often disappear or reappear instantaneously. On! Off! The situation becomes interesting when the occultation occurs near the Moon northern or southern limb. In such cases we can observe the star disappearing behind mountain peaks and reappearing in the valleys between the mountains.
The so-called grazing occultations are the caviar for the occultation observer!

• The type of event
• The altitude of the Sun (the daytime does spoil a lot...)
• The brightness of the objects involved
• The location on Earth from where you're observing
• The size of your telescope
• The experience you have as an observer

Most occultations observed are those of stars occulted by the Moon. Every year some 7000 of these events are witnessed by observers around the globe and the events are timed.