Here is a link to a good description of Discussion vs Dialogue. I found this via the DialogueMapping Yahoo! Group mail list. There are a number of good tips in the article (provided by The Henderson Group) so you should have a look at the link, but here is the description provided at the end:
Dialogue Contrasted With Discussion
Discussion has the same roots as “concussion” and “percussion.” The Latin origin of discuss is “discutere” – to dash or shake apart. Hence, to discuss is to shake apart what others say.
In a discussion we break things down, fragment the whole, analyze the pieces, and seek to convince others of our insights. You recognize discussion by its competitive nature. If you are only listening in order to prepare your own counter-arguments, you are involved in a discussion.
Defaulting to Discussion
Often the default in business conversations, is discussion. Each side will lob its viewpoint across the table. The other will then repeat its counter-position. You have a sense of positions being smacked back and forth like a puck in a hockey game.
If your trust of the others involved diminishes along with your patience and good will, you are likely in discussion.
There are a lot of ways you can structure dialogue. One way we use a lot in Competitive Thinking is the Business War Game, where each participant group provides not only a position statement but also must provide relevant factual backup and reasoning. In this way the tendency to deconstruct or counter an argument is minimized and the focus is on the validity and applicability of the underlying reasoning.