Depends on where the ADR is. A few places (like the dining shows, the castle etc) take a credit card as a deposit against no shows. I'm fairly certain you are charged at least a portion of the price.
Other ones, no, you're not charged, but please do be a doll and cancel even 5 minutes before, so someone else has the chance to eat there
No, they don't charge you for missing an ADR. The ones that do are the ones that require the credit card when you make your reservation.
And like the other post mentioned, please, please try to cancel any ADRs you don't need even if it is at the last minute. Many reservations are so hard to come by.