A twin bed means that you will have two separate beds in your hotel room. Twin beds could mean two singles, two doubles or two queens depending on the bedding configuration and availability at the hotel.
A double bed means that you will have one bed in your hotel room. This could mean that you could have a double, queen or king bed depending on the bedding configuration and availability at the hotel.
Bedding configuration is not guaranteed at any time as rooms will be allocated during check-in.