To my understanding, the previous replies are valid for Diners Club US cards issued by Citibank. In other countries the license for Diners Club brand has been given to other banks.

For example here in Finland the card is issued by SEB Kort. Here the DC card is not Mastercard, but it works as Mastercard when using the Finnish card in US/Canada.

The annual fee varies from issuer to issuer, as well as the benefits from the Club Rewards program.

ps. as a general comment to fellow club members, I'd like to ask you to mention the country or the issuer you are talking about.