Explanation/Reference:
A service-level agreement ( SLA) is a contractual agreement outlining a specific service commitment made between contract parties -- a service provider and its customer. The SLA includes language describing the overall service, financial aspects of service delivery, including fees, penalties, bonuses, contract terms and conditions, and specific performance metrics governing compliant service delivery. These individual performance metrics are called service-level objectives (SLOs).
References: http://searchcio.techtarget.com/answer/Whats-the-difference-between-SLO-and-SLA