Layers are handled in one of two ways.

Manual Layers

The first way is manually creating the layer and all the options in the full map array. Here is an example:
'layers' => array(
  'default_wms' => array(
    'id' => 'default_wms',
    'type'=> 'WMS',
    'name' => t('Default Open Layers WMS'),
    'url' => 'http://labs.metacarta.com/wms/vmap0',
    'params' => array(
      'layers' => 'basic',
    ),
    'options' => array(),
  ),
),

Named Layer

The second way is to use the a named layer provided by the hook_openlayers_layers_info() (see the OpenLayers hooks). For example:
'layers' => array(
  'openlayers_default_wms',
),

Basic Layer Properties

Layer properties are unique to the type of layer. Below are the common properties. TODO: Add docs for the specific options for each layer that OL modules provide.