A fully managed enterprise messaging service with queues (point-to-point) and topics (pub/sub with filtered subscriptions). It supports transactions, sessions, ordered delivery, and dead-lettering.
On AZ-305 you choose Service Bus when you need enterprise messaging with strong guarantees (ordering, transactions) as opposed to Event Grid (lightweight reactive events) or Event Hubs (massive telemetry streaming). Classic question: process orders reliably and in order between microservices → Service Bus.
Azure Service Bus is a concept that can show up on the exam. Check the full guide to see it in context.