Engineering managers need to read this.

Best explanation of the differences I've seen.

SREcon keynote speaker Ben Treynor (founder of Google's SRE team) laid it out clearly:

Site Reliability is what happens when a software engineer is tasked with what used to be called operations.

In a recent interview, he explained further:

To SRE, software engineers are people who know enough about programming languages, data structures and algorithms, and performance to be able to write software that is effective. Crucially, while the software may accomplish a task at launch, it also has to be efficient at accomplishing that task even as the task grows.

In other words, when infrastructure is code, you only hire people who write code. And that makes a lot of sense, especially for anyone who operates at web-scale.