A discussion is open to all members of a class or assigned to a group, whereas a journal is personal between the student and the tutor.
The most common form of interaction in an online course is through discussions started by an instructor. Participation and interaction in discussions don't occur naturally. You need to intentionally design it into your courses. To encourage engaging discussions, craft discussion questions carefully, and create enquiry.
Journals are personal spaces for students to record their thoughts, processes and to self-reflect. They are shared between the student and the instructors on the site - but are not viewable by the other students on the course.
You can create journal assignments that are broad and student-directed. Students can reflect on the learning process and document changes in their perceptions and attitudes. Students can describe the problems faced and how they solved them.
Journals are ideal for individual projects. For example, in a creative writing course, each student creates entries and you provide comments. In this manner, a student can refine a section of a writing assignment over a period of time, with your guidance and suggestions. Students can also comment on their own entries to continue the conversation.