See the class list to the right and especially take a look at {@link ol.Map} and {@link ol.layer.Layer} because those are the central objects.
In general every use of OpenLayers starts by initializing a map, then adding the required layers. Controls and interactions can be added to change the behavior of the map.
A {@link ol.proj.Projection} defines which point on earth is represented by a pair of coordinates. Coordinates within OpenLayers can be used in various projections where some common projections are always supported, others can be used via Proj4js.
A map in OpenLayers is essentially a staple of layers that is viewed from the top. Layers are responsible for retrieving data and displaying it.
See CONTRIBUTING.md for instructions on building and testing OpenLayers. The file does also describe how to commit your changes to OpenLayers.