An SLA is an agreement between a client and a service provider on the level of service that the provider must provide.
However, the SLA isn’t usually a separate contract between the client and provider.
Let’s say someone needs internet access in their new apartment so they sign a contract with an internet service provider. The contract will include language covering all aspects of the service being provided, like the specific services included in the package, the price, the speed of the connection (bit/rate), the availability of the internet (99.999% uptime), and the business and technical support responsibilities of the provider.
This is the service contract between the person and the internet service provider. Now, the SLA isn’t a section in this service contract. The word SLA usually isn’t even included in the contract. The term SLA is used off-paper to encapsulate particular aspects of the agreement, including quality (internet speed), availability (uptime), and responsiveness (availability of customer support, mean time to repair failures) agreed to between the customer and service provider.
In summary, the SLA is not an actual contract that is signed between customer and provider. It is just an off-paper term used to encapsulate the quality, availability, and responsiveness components of the service contract.