Yes. You can have a workflow rule without Alerts, Tasks and Assignment Values, but remember that no action will be performed when the workflow is executed. So it is important to associate a Workflow Alert or Task or Assignment Value for each Workflow ...
Workflow Rules Workflow Rules help you specify when a workflow should be triggered & when it should be executed ( based on criteria ). For more details about Workflow Rules, refer http://www.zoho.com/support/help/workflow-settings.html#wf-rules ...
Yes, you can update a particular field using WorkFlow Assignment Values. Goto Setup > Workflow Settings > Create a workflow rule. Then create a workflow assignment value and associate that to the Workflow Rule. For detailed help to create workflow ...