What is a message queue and when would you use one?

Message queues enable asynchronous communication between services by storing messages until consumers process them. Benefits: decoupling (producers/consumers independent), buffering (handle traffic spikes), reliability (messages persist until processed), scaling (multiple consumers). Use cases: order processing, sending emails/notifications, video processing, event streaming. Examples: RabbitMQ (traditional queuing), Kafka (event streaming), SQS (cloud). Essential for building resilient, scalable systems.