SLAs measure the KPI against the service resources purchased by customer. Resources can be number of hours, cases, car service, electrician service calls or any thing.
While Entitlements are customer contracts, it defines the terms and conditions of the contract like number of hours purchased or number of cases purchased.
For example, when a customer purchases a car, he can have an agreement with the automobile company of having one car service with in 6 months of purchase.
The SLA could say that, automobile company should book a car service within 6 months and service should be done in 2 days. The purchased terms and conditions are set in the entitlement, and service KPI is measured against the SLA which is 6 months and 2 days.