What is the primary difference between autoresponder and workflow use?

What is the primary difference between autoresponder and workflow use?

I've read the information about automation and I'm trying to understand what the a difference between autoresponder and workflow is...they seem to both get the same job done.
If I use autoresponder do I ALSO use workflows for the same campaigns? I would be planning to do an autoresponder based on Sign Up, with 3 messages total.