Transforming a point means you take a coordinate in one projection and turn it into a coordinate of another projection. This operation is also called reprojection. The term reprojection is also applied when deforming a raster image from one projection to another. Apart from transforming EPSG:4326 to EPSG:3857 and vice versa, OpenLayers does not provide support for transforming other projections out-of-the-box. To do transforms with other projections, you can include Proj4js (which can be found at http://proj4js.org) or provide your own transforms and register them with OpenLayers. You may be wondering why. The main reason is to not maintain projections in the core library when we can keep it outside, in a well-maintained library. The other goal, is to avoid increasing the overall library size. There are thousands of projections, whereas most projects only require some of them. The gain is not worth the drawback.

In most scenarios, it is the job of the backend map server to handle projection transformations. Often, it's useful or faster to do it on the client-side (such as in the case of vector layer coordinate transformations) because we don't need to call server-side again for transformation or because we don't control the server-side like for external web services. Let's take a look at how to transform EPSG:4326 to EPSG:3857 with OpenLayers.