Most forms of projection operate by projecting Earth coordinates onto a geometric shape that can be easily flattened to a twodimensional image. Three geometric shapes are frequently used:
Cylinder 
Earth coordinates may be projected onto a cylinder. The cylinder is cut lengthwise and unrolled to make a twodimensional map. This type of projection is called a cylindrical projection. Some characteristics of cylindrical projections include the following: 



Cylindrical projections in MapViewer are: Cassini, Equidistant Cylindrical, GaussKruger / Gauss Conformal, Hotine Oblique Mercator, Mercator, Miller Cylindrical, Oblique Mercator, Transverse Mercator, and Universal Transverse Mercator. 
Cone 
Earth coordinates may be projected onto a cone. The point of the cone is usually directly above the pole and the sides of the cone pass through the globe at two userdefined latitudes, called the Standard Parallels. At the standard parallels, there is no difference between the eastwest and northsouth scales. The cone is cut from tip to base and unrolled to make a twodimensional map. This type of projection is called a conic projection. Some characteristics of conic projections include the following: 



Conic projections in MapViewer are: Albers Equal Area, Equidistant Conic, Lambert Conformal Conic, Polyconic, and Bonne. 
Plane 
Earth coordinates may be projected directly onto a flat plane. This type of projection is called an azimuthal projection. Projections of this type are recommended for maps of polar regions because cylindrical and conic projections generally either have severe distortion in polar regions or are unable to project coordinates in polar regions. The most notable characteristic of azimuthal projections is that the side of the Earth that is facing away from the center of the projection is not visible.
Plane projections in MapViewer are: Azimuthal Equidistant, Gnomonic, Orthographic, Stereographic, and Lambert Azimuthal Equal Area. 
Other 
Projections in this category are pseudocylindrical, pseudoconic, or based on some other mathematical projection or mathematical tables.
These projections include: Eckert IV, Eckert VI, Mollweide, Robinson, RobinsonSterling, Sinusoidal, State Plane, Unprojected Latitude/Longitude, and Van der Grinten. 
* The State Plane Coordinate System uses Transverse Mercator, Lambert Conformal Conic, or Hotine Oblique Mercator, depending on the zone.
See Also
Introduction to Map Projections
Characteristics of Projections
Latitude/Longitude Coordinates
Latitude/Longitude in Decimal Degrees
Using Scaling to Minimize Distortion on Latitude/Longitude Maps