While fine-tuning a technical workshop for next week, my co-presenter and I realized that our comment of "don't forget these knowledge base articles!" (see terminology note below) wasn't very helpful. You used to be able to go to http://support.esri.com and type in a knowledge base/technical article number like 23025. That would bring up "FAQ: Projection Basics: What the GIS Professional needs to know."
The Support website has been redesigned and that trick no longer works. Below I've included the new links to some of the useful coordinate systems, map projections, and geographic (datum) transformations technical articles.
23025: http://support.esri.com/technical-article/000005562, FAQ: Projection Basics: What the GIS professional needs to know
17420: http://support.esri.com/technical-article/000002813, FAQ: Where can more information be found about coordinate systems, map
projections, and datums?
29129: http://support.esri.com/technical-article/000007880, How To: Identify the
spatial reference, projection, or coordinate system of data
21327: http://support.esri.com/technical-article/000004829, How To: Select the correct geographic (datum) transformation when
projecting between datums
24893: http://support.esri.com/technical-article/000006217, How To: Identify an unknown projected coordinate system using ArcMap
How To: Define the projection for CAD data for use in ArcMap
If you have a favorite KB/technical article number, you can also append the number to this partial link:
which will automatically redirect to the new URL.
Terminology note: Originally, they were called knowledge base articles and sometimes referred to as KB articles. A few years ago, the name was changed to technical articles.