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Azure Service Bus is a managed service for building reliable and connected applications (either on-premises or in the cloud) leveraging messaging patterns. Service Bus is often used as a key component in eventually consistent solution architectures—providing asynchronous messaging integrated with additional Azure resources such as SQL Database, Storage, Web Apps for App Service, or applications hosted on Azure Virtual Machines.

Service Bus features four different communication patterns:
 Queues Provide a basic FIFO (first in, first out) messaging pattern. Messages in a queue are stored until they are retrieved and deleted. Service Bus queues are conceptually similar to Azure Storage queues, yet they offer a few more advanced middleware capabilities (dead lettering, auto-forwarding, sessions, duplicate detection, etc.).

 Topics Provide a publish-and-subscribe messaging pattern. A message can be written to a topic, and multiple subscriptions can be attached to that topic, with different subscriptions receiving different messages depending on a filter.

 Relays Provide a bidirectional (two-way) communication pattern. Instead of storing messages like queues and topics, a relay simply proxies calls from the client/producer to the server/receiver. Service Bus Relays is also one of the older services in Azure. It was publicly announced in May 2006 as Live Labs Relay (since incorporated into the Service Bus family).

 Event Hubs Provide a highly scalable event and telemetry ingestion service for supporting scenarios requiring low latency and high reliability. The section below discusses Event Hubs in more detail.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition