A coordinate is a set of numbers that uniquely identifies a location. Coordinate systems provide the mathematical framework for describing where things are on Earth.
Geographic Coordinate Systems
A geographic coordinate system (GCS) uses angular measurements — latitude and longitude — to describe locations on the surface of the Earth. The Earth is treated as an ellipsoid, and positions are measured in degrees from reference lines.
Projected Coordinate Systems
A projected coordinate system (PCS) takes the curved surface of the Earth and projects it onto a flat plane, using linear measurements (metres or feet) instead of degrees. This makes it easier to measure distances and areas accurately, but introduces distortion.
Datums
A datum defines the shape and size of the Earth model used as the basis for a coordinate system. Different datums produce slightly different coordinates for the same physical location. The most common global datum is WGS84, used by GPS. In Canada, NAD83 is the standard for government spatial data.
Mixing data from different datums without converting between them can introduce errors of several metres. When combining spatial datasets, always check that they use the same coordinate reference system (CRS).
EPSG Codes
Coordinate reference systems are identified by EPSG codes. WGS84 is EPSG:4326. Web Mercator is EPSG:3857. NAD83 Statistics Canada Lambert is EPSG:3347.
Learn specifically about Latitude and Longitude.