This type of Action allows to send an email in different formats.

• Add an action with the Action Type = Send Email (1). See more info about creating Actions in Working with Actions article
• Select users to send emails to (2)
• Specify "CC" field value, if needed (3). Here you can add static email address, use Merge fields and Formulas. 
• Specify "From" field (4).
  By default user will get email from current user email address. If you specify "From", user will get email from current user email address, but the label will be shown as specified in "From". If the Organization has "Organization-wide email address" specified, you can insert it's name in the "From" field and it will be displayed to the recipient as "from" address. 
• Populate mandatory "Subject" field (5)
• Select email Type (6). See below.
• Specify Attachments (7), if necessary. See below.

Email types are:

• Plain Text - email type using a very basic character set and contains no formatting or images. The email message should be typed in the proper area (9).
• HTML - email type supporting HTML to provide formatting and semantic markup capabilities in email that are not available with plain text. The email message should be typed in the proper area (9) as HTML code. If HTML is selected, a "Preview" button (11) will be shown. Click it to see how HTML code entered will be shown in browser.

• Email Template - user is to choose existing email template from the list.

If select this type, you must select Email Template (12). Note that to be able to use standard email templates, you should provide "Email Delivery Contact" on the Notification Settings Tab.

Optionally, specify the Source ID (13). It is an ID of the record of Data Source Object, whose data will be used in Email Template merge fields (if any). If not specify (by default) it will be an ID of the current Action.

• Execute in background (14): select it if you need to execute this action in other context than current user operation. See more info at How to use Execute in background option. If check this option, the new field will appear:
• With gap (15): by default it is 0, but can be set as an integer number of minutes. This field specifies interval before action execution.

• Adding attachments (7). This option is available only if Email type = Plain text or HTML:

• Add as many attachments as necessary
• Select Record or Process in the "Type" field (16)
• Enter File IDs or Names if you have source record specified. All files with matching names attached to the source record will be sent as attachment. For IDs you should specify Content Version IDs.
• If you select a Process in the "Type", all files with matching names attached to the step, which action has been executed at, will be sent as attachment.