I've always assumed each number delineated by a period represented a single component of the software. If that's true, do they ever represent something different?
How should a version number be structured to start assigning versions to the different builds of my software? As an aside, my software has five distinct components.
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.