A point cloud is basically an array of point data within a GIS which are used to describe or represent a surface of some kind. For example, a three-dimensional object can be portrayed via the use of a point cloud where the points represent vertices of a polygonal surface. The point cloud can be created with a 3D scanner or generated on a computer mathematically.
The points in a point could also represent elevation points on the earth's surface. In the case of elevation points, they can be expressed in two-dimensional space or three-dimensional space, and can come from survey data, LIDAR data, or even stereo photography. All point clouds are referenced to specific coordinate systems.