Which is the correct OpenLayers JavaScript to import?

Question

In the OpenLayers examples online such as this one: https://openlayers.org/en/latest/examples/accessible.html

I see them import the following script:

<script src="https://openlayers.org/en/v4.6.4/build/ol.js"</script>

However, in many places online such as this SE question: OpenLayers take user input of XY and create a point

I see them use this script for open layer:

<script src="http://openlayers.org/api/OpenLayers.js"></script>

What are the differences? Is it just a version difference? Why is one called ol.js and the other OpenLayers.js?

Answer 1

What are the differences? Is it just a version difference?

It is just a version difference as you suspected. http://openlayers.org/api/OpenLayers.js is for OpenLayers 2.x.

There was a rewrite at OpenLayer 3, and the .js file gets renamed to the shorter ol.js. (And so are the variables etc.)

There are many differences between openlayers 2 and 3+. Among them, some of the openlayers 2 functions are no longer provided in OpenLayers 3+ (due to a change of philosophy). See e.g. Missing LayerSwitcher in OpenLayers3? and other GIS.SE questions.