SAP PI (Process Integration) is SAP's middleware product that acts as an enterprise service bus (ESB) and uses a message-oriented approach to facilitate integration across systems. It provides design-time governance features and supports integration through various adapters. Some key uses of SAP PI include suppliers sending invoices to ERP via XML, CRM triggering refunds to customers in ERP via web services, and aggregating production stats messages and sending to ERP. The runtime environment includes components like the integration server, enterprise services repository, and integration builder for routing rules.