Cloud functions work much the same way as cloud actions, except that they don't perform an operation on a specific instance, and so do not need a machine name to be specified. However, since they perform an operation on a specific cloud provider, that provider must be specified.
list_nodes: Returns some general information about the instances for the given provider.
list_nodes_full: Returns all information about the instances for the given provider.
list_nodes_select: Returns select information about the instances for the given provider.
Another useful reference for viewing salt-cloud functions is the Salt Cloud Feature Matrix.