Azure Service Bus is a cloud-based messaging service provided by Microsoft Azure that enables reliable communication between different applications and services. It allows for the exchange of messages in a decoupled manner, meaning that the sender and receiver do not need to be online at the same time. This is particularly useful for distributed systems where components may be running on different servers or in different locations. The service supports various messaging patterns, including queues for point-to-point communication and topics for publish-subscribe scenarios. With features like message durability, automatic scaling, and security, Azure Service Bus helps ensure that messages are delivered reliably and efficiently, making it a vital tool for building scalable cloud applications.