The following article refers to functionality available in Workflow builder, which can be accessed in one of the following locations, depending on your configuration:

Administration > HR > Workflows 
or
Administration > HR > Workflows > Workflows

When adding a step to an approval workflow (any workflow where the Workflow Category is set to Approval), there is an option to make the step Notify only:

When this is set to No then the step will not generate an approval request for the recipient, but rather send a message to the recipient.

It might appear that this does exactly the same as sending an email using a Send email to specified recipient step, but there is actually an important difference.

When an approval is declined, all steps which are marked as Notify only will still trigger.  The email will be delivered to the specified recipient, whereas steps with a type of Send email to specified recipient or Send email to (external) email address will not.

This is useful so that you can have different notifications going to different people depending on whether the request is approved or not.  You can use a Notify Only step to notify anyone who might want to know the outcome (whether approved or declined) and use a Send email to specified recipient or Send email to (external) email address to notify people only if the request was approved.  

Please note that the ordering of steps can have an impact on this as can the conditions indicated in When should this happen? but, generally, the above will be true in the majority of cases.

Automatic Approval

It is possible to create an approval workflow which automatically approves the request.  To do so, the workflow should contain only Notify Only steps and not have any Require Approval steps.  In this case, the emails will be sent from the Notify Only step and the request will be automatically approved when the workflow completes.

Further Resources

A training video on the topic of building approval workflows can be found here.