How do you tell the difference between a discussion and a debate...in one word:  rebuttal.

In a discussion, when you disagree with someone you will probably tell them them your opinion, perhaps bolster that position with some "evidence" and then given them a chance to reply.  In a discussion, your "opponent" (for lack of a better word) then has the option to present his (or her) opinon in the matter..citing such evidence as they wish.  After you both have a chance to express your opinion, the discussion phase ends...either with a concensus (or the tacit agreement to "respectfully disagree) on the matter.

In a debate, however, the participants take the "discussion" to the next level, and instead of trying to understand the other person's perspective (and agreeing to disagree), they try to demolish each other's  argument by systematically refuting their "evidence" and thereby calling their interpretation into question. 

Frankly, that's why most debate and forensics competitions are governed by a fairly strict set of rules.   When it's all said and done, a debate *is* a competition, it's easy to get caught up in the "win/lose" trap and start attacking your opponent (instead of his or her argument).

Does that help explain it any?