What do the numbers in a version typically represent (i.e. v1.9.0.1)?
In version 1.9.0.1:
1: Major revision (new UI, lots of new features, conceptual change, etc.)
9: Minor revision (maybe a change to a search box, 1 feature added, collection of bug fixes)
0: Bug fix release
1: Build number (if used)—that's why you see the .NET framework using something like 2.0.4.2709
You won't find a lot of apps going down to four levels, 3 is usually sufficient.
It can be very arbitrary, and differs from product to product. For example, with the Ubuntu distribution, 8.04 refers to 2008.April
Typically the left most (major) numbers indicate a major release, and the further you go to the right, the smaller the change involved.
There is the Semantic Versioning specification
This is the summary of version 2.0.0:
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.