Discussions is a simplified form of a Forum.
Discussions can be created by instructors (or even learners according to the privileges of their user-type). To create a new discussion, click on the "Add discussion" link on the right hand side bar (1) as Instructor.
In the pop-up fill the topic title and its description and check the "Private" option if you wish to make this topic available only to users that know its URL. Then click on the Post button to create the topic.
If the topic is not private, it will be shown in the list of discussions that are currently active. Anyone can reply to a specific comment to create the visual feel of a conversation (1), or can reply to the original post creating a new comment (2).