elrepo
-----------
This is the main channel and is enabled by default. As this channel should not contain packages also present in the distribution, it should be safe to run a 'yum update' with this repository channel enabled.

elrepo-extras
-------------
The elrepo-extras channel provides packages and their dependencies that replace/update RHEL distribution packages.

elrepo-kernel
--------------
The elrepo-kernel channel provides both the long-term support kernels (kernel-lt) and the latest stable mainline kernels(kernel-ml) using sources available from the Linux Kernel Archives.

#yum --disablerepo="*" --enablerepo="elrepo-kernel" list available
#yum --enablerepo=elrepo-kernel install kernel-ml

There is no need to install the kernel-ml-headers package. It is only necessary if you intend to rebuild glibc and, thus, the entire operating system. If there is a need to have the kernel headers installed, you should use the current distributed kernel-headers package as that is related to the current version of glibc.