Let's jump in to using rules and filters. Add a WMS layer to the map. We'll also need a vector layer, so add in a vector layer:

        vector_layer = new OpenLayers.Layer.Vector('Basic Vector Layer'); 
        map.addLayer(vector_layer);

Next, we'll need some features. We'll generate some random feature points again, and we'll create a property for each object called population which contains a random number between 0 and 2000:

for(var i=0; i<20; i++){
  vector_layer.addFeatures([new OpenLayers.Feature.Vector(
    new OpenLayers.Geometry.Point(
     (Math.floor(Math.random() * 360) - 180),
      (Math.floor(Math.random() * 180) - 90)
    ),
    {
      'population': Math.floor(Math.random() * 2000)
    }
  )]);
}

Now we need to create a style object—rules are applied directly to style objects, and filters are applied directly to rules. So, let's create...