Map services are maps made available to the web using ArcGIS. A map is created in ArcMap and published as a service to an ArcGIS Server site. Internet or Intranet users can then use the map service in web applications, ArcGIS for Desktop, ArcGIS Online, and other client applications.
To access the individual feature layers within a map service, the layer id must be referenced and added to the ArcGIS Online web map. For example, https://apps.fs.usda.gov/arcx/rest/services/EDW/EDW_RightofWay_01/MapServer/0 will add the EDW Right of Way feature layer to your web map.
For a more in-depth walkthrough of how to use EDW Map Services, please refer to the following guide: How to Use Map Services
How to see whether certain data layers are available for a specific forest:
Some data is published on a forest by forest basis. The instructions in this PDF document will tell you how to determine whether certain data layers are available for a specific forest.