You never can have enough sources on this topic. Here are

SharpGIS - Spatial references, coordinate systems, projections, datums, ellipsoids – confusing?

People are often mixing the above as if they were one and the same, so here’s a recap of them. One of the things you often find people saying is that “my data is in the WGS84 coordinate system”. This doesn’t really make sense, but I will get back to this later.

On Coordinate Systems

If you feel like ending a conversation with a developer then simply bring up the topic of character encodings.

Also:

• A JavaApplet that demonstrates different map projections.
• The Spatial References website, which lists projections information in various formats.