You are better to read some basic geography to understand these concepts.
A systematic transformation of the latitudes and longitudes of locations on the surface of a sphere or an ellipsoid into locations on a plane. Maps of places on earth for 2-D view needs projection, if you want to project something to display in a Cartesian (x,y) coordinates system, you need an origin, scales to define the distance, and bounds; and that’s the job for Spatial Reference. For a 3-D view, you also need z value, i.e. Elevation.
A Spatial Reference System (SRS) or Coordinate Reference System (CRS) is a coordinate-based local, regional or global system used to locate geographical entities. A SRS defines a specific map projection, as well as transformations between different spatial reference systems. Like a date-time reference for time, i.e. Greenwich Mean Time. There are many spatial references defined by various organizations and religions, please see in Wikipedia. For example: EPSG 4326 for the current GPS measurement.
Geographic Coordinate System (GCS)
You can think of a Geographic Coordinate Systems as data that is defined by a 3-D surface and measured in latitude and longitude. An example of a Geographic Coordinate System would be “WGS 1983” or “North American Datum 1983”. You may also wonder what a “Datum” is. Just remember that the term “Datum” and “Geograhpic Coordinate System” can be used interchangeably. Essentially a Datum provides a “frame of reference for measureing locations on the surface of the earth i.e. lines of latitude and longitude.”
Projected Coordinate System (PCS)
A projected coordinate systems refers to data that is defined by a flat 2-D surface and can be measured in units of meters and feet. An example would be USA Albers Equal Area Conic which has a measuring unit of Meters. “Map projections” and “Projected Coordinate Systems” can be used interchangably as well.
Read the a ArcGIS content for a fully understand.