This is a great question. A debate is a method of argumentation based on a topic that is usually controversial. On a more formal level, a debate usually has two people debating a topic at a forum with an audience. More importantly, there is a moderator who oversees the debate to ensure equality and order.
Typically each person in the debate has prepared statements, which he or she can read. After this, there is a time for rebuttals that each debater can give. Then there is usually a time for each person to respond and finally there are closing statements. Of course, the format can differ and usually it does based on many different considerations. With this stated, there might be a time for questions from the moderator and the audience.
Debates are also timed, so that there would be parity in terms of time given to each debater.