Ideally those can be the same thing; see the current Mailman interface, or Discourse, both of which can provide a forum-like interface to a mailing list.
Forums are more for new and normal users/fans of open source projects. Technical discussions happen over IRC. And even more longer insightful technical discussions (often over several days) happen on mailing lists.
Ideally those can be the same thing; see the current Mailman interface, or Discourse, both of which can provide a forum-like interface to a mailing list.