Landslides refer to the downward movement of land masses along steep slopes under the influence of gravity. They are classified based on speed and type of movement. Slow movements include solifluction, creep, and soil creep. Rapid movements are rapid flows. Landslides occur due to internal causes like steep slopes, groundwater, and lithology as well as external causes like earthquakes. Preventive measures include controlling slope angles, installing drainage systems, and stabilizing weak geological structures.