A topic and queue use different messaging delivery patterns; each one can be configured in a Message Queue service.
Topic. When a message producer sends a message to a topic, a publish/subscribe architecture is used. In this broadcast pattern, a producer sends a message to a topic destination. Any number of consumers can be subscribed to this topic destination. Each consumer subscribed to the topic gets its own copy of the message. If no consumers are subscribed to the topic, the message is discarded.
The Event Notification Service also uses a publish/subscribe architecture; it is similar to the topic pattern defined in Message Queue.
Queue. When a message producer sends a message to a queue, a point-to-point architecture is used. In this pattern, a producer sends a message to a queue destination from which only one consumer can receive it. If several consumers are waiting for messages from the queue, only one subscriber will receive the message. If no consumers are waiting, the message is held until either the message times out, or a consumer expresses an interest in the queue.
Producing messages to a queue allows you to spread the message load across multiple consumers.