The diagram below (also available as a PDF) is my interpretation of how the three staple tools of elearning are best suited to different tasks. This overview comes from the technology angle (rather than more correctly fro the learning objectives), however it might be useful in differentiating to colleagues the strengths of each of the three main collaborative tools, once a particular activity has been designed.
What tasks suit what tool?
Blogs are good for: individual reflection, reporting back activity and logging activity.
Wikis are good for: producing a solid piece of work, a sharable space for groups, longer writing/website creation.
Discussion boards are good for: question and answer, multiple discussions on different topics/activities, offers very structure discussions.
What are the common outcomes of using these tools?
Blogs and wikis allow easy and simple group collaboration.
Wikis and FAQ-type (clearly threaded) discussion boards can be easily used as a resource for revision by all students (i.e. multiple groups viewing each others’ work).
Topic-based discussion boards and blogs allow discussion or sequential development of ideas.
The most important point
All tools provide opportunity for providing feedback to students.