What Linux kernel exactly is? And does Ubuntu, at the core, use a modified Linux kernel? And in what manner is it distinguished by the NT/Windows 8 kernel?
Ubuntu Linux kernel derives its kernel from the vanilla kernel d(using linux.git and stable branches) and builds its own version of supported kernel.
Modified kernel is a user requirement for building a custom kernel from the sources (either from upstream or ubuntu sources) for testing a new device driver etc.
My 2 cents! Hope this helps.