The Extension Suite is a strategic approach for extending your cloud, on-premise, non-SAP, and SAP business solutions beyond what your core system provides.
SAP Extension Suite gives you the services, tools and capabilities to build and maintain extensions to your business apps in the cloud. Use it when you need to implement additional processes on top of your existing SAP applications.
The Business Technology Platform, or BTP for short, is a cloud-based PaaS (Platform as a Service) offered by SAP. The BTP provides application developers with a canvas where they can create applications and extensions to suit their customers customization needs.