What is the difference between Event Receiver and Workflow?

Event Receivers Workflows
Cannot be initiated manually Can be initiated either manually or automatically
Event receivers can be either Synchronous or Asynchronous Workflows are always asynchronous, i.e. workflows get executed after the action/operation
User interaction is not possible in event receivers You can use various forms like initiation form, association form for user interaction.  
You can cancel running synchronous(before committing to Content Database) event receiver You cannot cancel workflow action as the change is already made in Content Database(workflow = always asynchronous)
Can be created only in Visual Studio Can be created using Visual Studio, Visio or SharePoint Designer
Event receivers execute on WFE, so if WFE reboots, event receiver may end abruptly without complete execution. Workflows are robust, can continue after WFE reboots. 
Event receivers run for very short period of time Workflows can run for days, weeks, months or even for years!!
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