The performance of a service is monitored against a Service Level Objective (SLO), which in turn relates to a Service Level Indicator (SLI).
The SLI tells you what is measured and how. The SLO tells you what the SLI target is for a particular service.
Here’s an example.
An SLI we’ll call P95 is the measurement of the 95th percentile Service Time value determined for each one-minute period and measured at the server using a network trace or web log value.
An SLO for any Application A page could be the P95 figure must be <= 1 second, whereas the same P95 SLO for an Application B page might be <= 3 seconds.
An SLA is a financial or legal consequence of failing to meet an SLO. So for operations, we care about SLOs not SLAs.