A debate is a generally a public discussion between individuals with differing views on an issue. In politics as usual, debates are usually dominated by rhetoric, spin and derogatory remarks, the object is not to enlighten the audience but generally to make them dislike the other participants. Generally, debates are a zero sum game in which for one person to win, others must lose.
Deliberation on the other hand, is a kind of debate where the participants are still willing to learn, be convinced to change their position, and seek to find agreement with each other. Deliberation and partisan politics are usually mutually exclusive, much to the detriment of civil society.
Open politics uses a style of online deliberation, called an IPA which discourages spin and rhetoric and tries to maximize the informative value of the debate for the audience and the participants.