Logic is a formal method of argument. Logical arguments generally take the form of "A therefore B" or "if A then B". The A part of the argument is called the premise, and the B part is called the conclusion. An extended argument may use conclusions from previous statements as premises in subsequent statements: A therefore B, B therefore C, C therefore D, etc.
An argument is considered logically sound if the conclusion does indeed follow from the premise. If the argument is sound, then the conclusion is true if the premise is true. Note that an argument can be logical even if the premise is false.
Logic is the preferred method of debating, but some debaters prefer to use fallacious logic to reach their preferred conclusions.