Although newsletters and emails are both used to send emails within marketing automation, they are used for two very different purposes. Newsletters are designed to be sent once to a specific set of people. Once you choose your conditions for a newsletter, it will be sent to one group of people, one time.
Emails on the other hand, are used to load into marketing automation workflows. What that means is that they are created and intended to be sent on an ongoing basis to whomever meets the conditions specified within your workflow conditions.
To find out more about how to create a newsletter, click here.
To find out how to create an email, click here.