What is the Vector Layer?

OpenLayers' Vector Class is generally used to display data on top of a map and allow real time interaction with the data. What does this mean? Basically, it means we can load in data from geospatial files, such as KML or GeoJSON files, and display the contents on a map, styling the data however we see fit. For example, take a look at this map:

This shows a map with a Google layer as underlying base layer and a vector layer on top of it. The data (all the circles with numbers in them) are loaded in from a GeoJSON file, an open file format that many other applications support. In the vector layer, there are a bunch of data points throughout the map. Each dot on the map is an object in the vector layer, and these objects are referred to as Features.

In this case, each feature is actually a cluster of data points—the numbers in each circle represent how many points belong to that cluster. This clustering behavior is something we can use out of the box with OpenLayers...