An 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 business
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.