Developers
Marketing
Campaigns, contacts, lists and segments.
Bulk sends to an audience, with the things bulk mail is required to have: unsubscribe handling, audience management and reporting. Marketing mail rides a separate egress path from your transactional mail, so a newsletter cannot damage the reputation carrying your password resets.
If you have used Mailchimp or Brevo, this is the part you know.
Guides
Start here. The reference tells you what the parameters are; these tell you what to do with them.
Reference51 endpoints
Bulk sends to a list or segment, scheduled and reported on.