What are major and minor numbers in Linux

In the old versioning system of Linux, it was: 

Major.Minor.Major Revision.Minor Revision. 

Odd number major or minor version numbers meant those were development releases. Think beta.

But now we're in a new system as of kernel 2.6:

2.6.Major.Minor 

So far it seems like the kernel developers have no interest in changing us to 2.8 or 3.0 at all, often citing the fact that changing those numbers is almost always just for marketing hype.
Also, odd number versions no longer mean they are in active development.

The Major version of Linux now releases every three months, usually introducing a lot of nifty new features, removing some, small updates to drivers, bug fixes, etc. You'll often find a lot of rolling release distributions will start developing new capabilities to take advantage of new stock kernel features and to remove features that aren't present anymore. Whereas with discrete Linux distributions it can sometimes take them months to years to provide a new kernel for their system, with the next release.