A judgment is a decision made by an arbiter (i.e. judge) stating whether a prediction is true or not.
When a prediction is submitted, it includes a judgment date as well as criteria for judging the prediction. An arbiter will issue a judgment that adheres to the acceptance criteria dictated for the prediction once the judgment date has passed.
Once a judgment is issued, there is a 24-hour period before it is finalized and karma awarded based on the judgment. During that time you can vote and comment on a judgment.
If you have evidence that shows that the judgment is incorrect, you can present it with your vote. If the arbiter is persuaded by your evidence, she may change her judgment.
It is possible that a prediction may be resolved before its judgment date, maybe even before its forecasting deadline has passed.
If this happens, the arbiter will be able to designate an "effective as of" timestamp with her judgment. Any forecasts occuring after that timestamp will be ignored.
In the event something like this happens, it will be the arbiter's responsibility to determine when the information resolving the prediction first became public. She will then set the timestamp for judgment at that time.