what is landslides ?

Answer 1

Wikipedia has a good summary:

A landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors affecting the original slope stability:

erosion by rivers, glaciers, or ocean waves create oversteepened slopes
rock and soil slopes are weakened through saturation by snowmelt or heavy rains
earthquakes create stresses that make weak slopes fail (see liquefaction, Hope Slide)
volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
vibrations from machinery, traffic, blasting and even thunder may trigger failure of weak slopes
excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures
groundwater pressure acting to destabilise the slope
in shallow soils, the removal of deep-rooted vegetation that binds the colluvium to bedrock.

Answer 2

Simple answer: when a whole bunch of earth moves from the top to the bottom! Most of the time, it takes everything in the way with it.

Think of an avalanche, but instead of snow, it's dirt, rocks, mud, etc. Landslides can happen above sea level or below.

Answer 3

in layman terms: landslide is a sudden fall of earth or rocks or combination of both down a slope, hill, cliff, etc.