What's the difference between the OS X version and build numbers?
System Version Number follows the MAJOR.MINOR.PATCH convention. From this site:

Given a version number MAJOR.MINOR.PATCH, increment the:

MAJOR version when you make incompatible API changes,
MINOR version when you add functionality in a backwards-compatible manner, and
PATCH version when you make backwards-compatible bug fixes.

Additional labels for pre-release and build metadata are available as extensions to the MAJOR.MINOR.PATCH format.

The number in brackets (14D136) is the build number (more info on the Apple site). This number is mainly for Apple developers, since normal users do not have access to non-public builds. In continuous integration, new builds can be made daily/weekly - every time the build number is incremented. When the developers/PMs (or anyone else in charge) is &quot;happy&quot; with the build, it can be released. Build numbers are constructed arbitrarily (I don't know of any standard numbering). It looks like Apple increments the first number by every MINOR release, then the letter with PATCH release and the last number is the real build number.