An occultation is when one celestial body passes in front of an other and blocks its view. Lunar eclipses, solar eclipses and jovian satellite eclipses are also considered occultations but most of the amateur work is done with the first two.
Lunar occultations happen when the moon passes in front of a star, major planet or minor planet.
Grazes happen when a star passes behind hills and valleys close to the moon's north or south pole. When this happens the star seems to wink on and off several times. The 'graze zones' are a mile or two from the edge of the lunar occultation path. Amateurs can observe and time lunar occultations to help astronomers refine their knowledge of the lunar topography and help map the lunar limb profile as well as helping with lunar orbit dynamics and stellar positions. Planetary occultations happen when a planet or asteroid passes in front of a star.