What is the difference between Azure IoT Hub and Azure IoT Central?

Differences between the two IoT offerings by Azure:

This page offers a good overview about the different IoT options on Azure: https://docs.microsoft.com/en-us/azure/iot-central/overview-iot-options

Microsoft Azure IoT Central and Azure IoT offer several options for building an IoT solution. These options are appropriate for different sets of customer requirements:

Azure IoT Central is a software as a service (SaaS) solution that uses a model-based approach to help you to build enterprise-grade IoT solutions without requiring expertise in cloud-solution development.

Azure IoT solution accelerators are an enterprise-grade collection of solution accelerators built on Azure platform as a service (PaaS) that help you accelerate the development of custom IoT solutions.

All of those build on IoT Hub which is the foundational PaaS (Plattform as a Service) offering for device connectivity, management and communication.