A SLA generally supplements a Services agreement. It’s an agreement that states the services to be provided and defines the level at which they should be provided.
There are three types of SLAs:
Customer-based SLAs: between a service provider and its customers
Internal SLAs: between two departments of a company
Multi-level SLAs: where there are multiple service providers and end-users, who can be customers or internal departments.
SLAs can be a formal legally binding contract or an informal agreement between parties, setting out the relationship in a given project (the latter is often used by public sector bodies).