Projection basics the GIS professional needs to know 

Release 9.2
Last modified November 9, 2006 
Print all topics in : "Getting started with map projections" 
Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for communication about a particular place or area on the earth's surface. The most critical issue in dealing with coordinate systems is knowing what the projection is and having the correct coordinate system information associated with a dataset. There are two types of coordinate systems—geographic and projected.
A geographic coordinate system uses a threedimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a spheroid). In a geographic coordinate system a point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads).
Learn more about geographic coordinate systems
A projected coordinate system is defined on a flat, twodimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid.
In a projected coordinate system, locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position.
Learn more about projected coordinate systems
When the first map projections were devised, it was assumed, incorrectly, that the earth was flat. Later the assumption was revised, and the earth was assumed to be a perfect sphere. In the 18th century, people began to realize that the earth was not perfectly round. This was the beginning of the concept of the cartographic spheroid.
To more accurately represent locations on the earth's surface, map makers studied the shape of the earth (geodesy) and created the concept of the spheroid. A datum links a spheroid to a particular portion of the earth's surface. Recent datums are designed to fit the entire earth's surface well.
The most commonly used datums in North America are:
Datum  Longitude  Latitude 
NAD 1927  122.466903686523  48.7440490722656 
NAD 1983  122.46818353793  48.7438798543649 
WGS 1984  122.46818353793  48.7438798534299 
Geographic coordinate system (Datum) Unit of measure Zone (for UTM) FIPS zone (for State Plane) Projection Projection parameters
1st standard parallel 2nd standard parallel Central meridian Latitude of origin False easting False northing Unit of measure