While the underlying technologies may be similar and used for similar purposes (ie to engage a user set/community in an interactive way), I think they differentiate in a few fundamental ways:
- Focuses on interactive discussions and comments between users. They often may be heated arguments between 1-2 users that end up taking up pages of this forum.
- Content is mostly organized and structured by topics/threads.
- It is best suited for fun/social topics where users love to spend hours on sharing their opinions
- Focuses (as the name implies) on answers to questions and the structure is streamlined for it.
- Take Quora as an example, where you can easily find an answer or post a question to the community.
- Content is organized around 'tags'
You find (in most cases) no heated debates about answers and that is intended.
Rather a good Q&A tool brings up top answers and rewards users with reputation points.