So far, we've been talking about the abstract idea of a projection. Let's dive into OpenLayer's
ol.proj namespace functions and the associated class
ol.proj.Projection class, which is what we use to actually handle projections. The
ol.proj.Projection class relies on internal code, managing the most used projection in the web mapping world: the EPSG:4326 projection (also named WGS 84) and the EPSG:3857 projection, also known as EPSG:900913 (using leetspeak, it means Google, the first company relying on this exact projection), and also, alternatively named WGS 84 Spherical Mercator. For reference, you can have the full history of the second projection at http://wiki.openstreetmap.org/wiki/EPSG:3857
For other use case ...