Will the candidate get an email when they are rejected?

This depends on whether the Administrator has set the workflow to automatically send a regret email to candidates when they are rejected.

When you click the Reject button, an information box will tell you if an email will be sent or not.

You can also check by looking at the workflow. Click the icon next to the candidate's App ID in the listing to show the workflow (a link is also available in the progress information box).