Styles define how features look on a map. Styles can be defined for the map as a whole, for specific events like when a feature is selected, or for individual features. Example style array:
array(
  'pointRadius' => 5,
  'fillColor' => '#FFCC66',
  'strokeColor' => '#FF9933',
  'strokeWidth' => 2,
  'fillOpacity' => 0.5
);
See the following OpenLayers docs as well:

Style Properties

See Style Properties for complete list of properties.

Context Sensitive Styling

TODO