Actually if you delete a domain databases will NOT be deleted since there is no way for DirectAdmin to know which database goes with which domain. If fact one database can be used with many domains.
Deleting a user account will delete databases associated with that user.
Deleting a domain will delete emails and email addresses associated with that domain. All your web files and logs will be deleted as well. Anything that was in /home/username/domains/example.com will be deleted.