When someone requests work, it needs to get done. The Service Level Management functionality specifies target times for how long a task should take and then uses Graphical Workflow to trigger notification reminders or perform automated actions. The results (success or failure) can also be reported or worked against.
The SLA (Service Level Agreement) engine in ServiceNow monitors effectiveness by comparing how long it takes to move between two conditions against a predefined goal. The main focus is on measuring time.
There is a field on the SLA called Type, with the choices SLA, OLA, and Underpinning Contract. These are only for reference and selecting one does not impact how the system works.
This definition may be different ...