What's the difference between Drupal 8.0.x and 8.1.x? On the drupal core download page I see two development branches, 8.0.x and 8.1.x.

I'm sure in the old days it just went linearly 8.0 then next would be 8.1 etc. but this suggests development is now parallel as if 8.0 and 8.1 are really different.

I'm interested to learn why this is this way. Why, for instance, as a developer would I work off 8.0.x and not 8.1.x?
The answer is semantic versioning. The "1" in 8.1.x, represents a branch that can contain new features as well as bug fixes. Backwards-compatibility to code written for 8.0.x is inherent in the numbering system. So, if your code requires features in 8.1.x, you should develop against it, but note you won't be able to deploy your code until 8.1.0 releases.