In OpenLayers, the vector layer is used to display vector data on top of a map and allow real-time interaction with the data. What does this mean? Basically, it means we can load raw geographic data from a variety of sources, including geospatial file formats such as KML and GeoJSON, and display that data on a map, styling the data however we see fit. For example, take a look at the map that follows:

It shows a map with a Bing satellite raster layer and a vector layer on top of it. The vector layer loads data using the OSM XML Vector source and draws it in real time with different styles based on the type and attribute of each feature (the individual points, lines, and polygons). In this example, a small subset of the OSM data has been requested and is styled by OpenLayers to highlight roads (white lines), parking lots (gray polygons), buildings (red polygons), green space (in green, of course) and the location of trees (the green dots). We'll cover vector styles...