What roles are there in a debate competition?

Answer 1

Well, it depends on what kind of debate you are discussing.

If you see a Presidential debate, you normally see two (or more) candidates squaring off, with questions asked by a moderator or panel.

In Lincoln-Douglas debates, you have:
Affirmative position debater - presents constructive debate points.
Negative position debater - cross-examines affirmative points. Negative position presents constructive debate points.
Affirmative position cross-examines negative points.
Affirmative position offers first rebuttal
Negative position offers first rebuttal
Affirmative position offers second rebuttal
[Thus you normally have one person on each "side."]

Finally, in CEDA or NDT debate you have :
1st Affirmative Constructive (2nd Negative Cross-Ex)
1st Negative Constructive (1st Affirmative Cross-Ex)
2nd Affirmative Constructive (1st Neg Cross-Ex)
2nd Negative Constructive (2nd Aff Cross-Ex)
1st Neg Rebuttal
1st Aff Rebuttal
2nd Neg Rebuttal
2nd Aff Rebuttal
[1st and 2nd indicating the two members on each team]

Note that in a competitive situation, there is also a judge [or panel of judges] to rate the debate and decide the winner. Often there is also a timekeeper, since the length of the speeches are strictly regulated.

Hope this helps!

Answer 2

Three roles: "for", "against" and "moderator".