The workflow web service provides following operations:
Returns all non-empty lists of the worklist with reference and name.
Returns the work list entries of the list defined by the reference parameter.
Returns a work list entry defined by the obj parameter.
Returns the work items for a work list entry defined by the obj parameter.
Executes a work item.
More information about the parameters of the operations can be found here (the listed actions are mapped to the corresponding web service operations):