A Service in ITSI represents a logical IT capability — like "Email System", "Payment Gateway", or "Customer Portal". Services are composed of KPIs that measure different aspects of performance and availability.
When designing services, you define ownership (who is responsible), dependencies (which services rely on which), and the KPIs that matter. Service dependencies create a tree: if a database service degrades, ITSI can show the cascading impact on the application and business services above it.