"kernel-headers" : Provides /usr/include/{ asm/*.h, linux/*.h, etc. etc.}.
See $ rpm -ql kernel-headers
"kernel-headers" : Are the headers that were used for compiling glibc.
Purpose : Used for compiling programs, not kernel.
"kernel-devel" : Are headers, used for compiling a single 3rd party module.
Updating the kernel to "Update level CentOS 6.7" : 10-Nov-2015 ...
# yum install kernel ( Or # yum udate kernel )
... which will also include goodies from later kernel versions, see `Changelog'.
Compiling a new kernel ( Not advisable ) : Use the kernel-source ...
kernel-2.6.32-573.8.1.el6.src.rpm , 10-Nov-2015
http://vault.centos.org/6.7/updates/Source/SPackages/
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