1. CRS provide a standardized way of describing locations. Many different CRSs are used to describe geographic data. The CRS that is chosen depends on when the data was collected, the geographic extent of the data, the purpose of the data, etc.
2. A Coordinate reference system defines, with the help of coordinates, how the two-dimensional, projected map is related to real locations on the earth. With the help of coordinate reference systems every place on the earth can be specified by a set of three numbers, called coordinates. In general CRS can be divided into projected coordinate reference systems (also called Cartesian or rectangular coordinate reference systems) and geographic coordinate reference systems.
Frame of Reference
Geographic Coordinate System, Projected Coordinate System
https://www.nceas.ucsb.edu/~frazier/RSpatialGuides/OverviewCoordinateReferenceSystems.pdf (1.); http://docs.qgis.org/2.0/de/docs/gentle_gis_introduction/coordinate_reference_systems.html (2.)
Author: Thomas D.