Confirmed Reservation  is a written or oral statement by the Hotel that they have received a Reservation and will honour it. Confirmed reservations may be either guaranteed or non-guaranteed.

Oral confirmations have virtually no legal worth, and even written or emailed confirmations have implied or specified limitations. For example, unless late arrival is specified, a hotel is not obligated to honour a confirmed reservation if the guest arrives after 6 pm.
Other definition:
Confirmed reservation is an oral or written statement by a supplier that he has
received and will honor a reservation. Oral confirmations have virtually no legal
weight. Even written or faxed confirmations have specified or implied limitations.
For example, a hotel is usually not obliged to honor a reservation if a guest arrives
after 18:00 (6 p.m.), unless late arrival has been guaranteed.