Simple definition
Site Reliability Engineering applies software engineering practices and service-level objectives to improve system reliability.
Purpose and use
SRE, Site Reliability Engineering, applies software-engineering methods to operations and service reliability.
How does it work?
Teams use service indicators and objectives to guide reliability, change and reduction of repetitive operational work.
Example and verification
Define an objective from the user's perspective and verify that measurement represents the experience actually delivered.
Points to watch
A highly available server does not prove successful user journeys. A 100% target can also conceal the absence of explicit reliability-versus-change tradeoffs.
In short
SRE connects automation, measurement and operational decisions to an agreed service level.