A Service Level Agreement (SLA) is a contract between a service provider (either internal or external) and the end user that defines the level of service expected from the service provider.
SLAs are output-based in that their purpose is specifically to define what the customer will receive. SLAs do not define how the service itself is provided or delivered.
The SLA will provide its customers is a basic example of an SLA from an external service provider. The metrics that define levels of service for an service should aim to guarantee: