[CentOS] CentOS, linux and kernel version

Thu Dec 19 07:29:22 UTC 2013
Jayadevan Maymala <jayadevan.technology at gmail.com>

I am using CentOS 6.4.
uname -r gives me

3.9.3-x86_64 Kernel version is 2.6.32. My questions -

What is 3.9? In Ubuntu, uname -r and /boot give the same version numbers.

Do we have a mapping of CentOS versions, the numbers like 3.9 and
kernel versions maintained somewhere?