We introduced the service level in a previous post. A Service Level can be defined/calculated for one service or a set of services.
Service Level Agreement
A contract with a service provider typically includes several services. An agreement about each of these services and their respective targets and quality parameters is called a Service Level Agreement (SLA). Often, though, the term SLA is used not to denote the contract as such, but rather the Service Level calculated for all the services is in the contract.