LKM

LKM stands for "Linux Kernel Module" or "Loadable Kernel Module". As the name implies, it is a way to allow code to interact directly with the kernel, in the kernel. In essence, it is a way to extend the kernel and add functionality to it. Although it goes without saying that you need root to modify the kernel, this opens up an interesting way to backdoor a box. You can see a list of currently loaded kernel modules in two ways:

$ lsmod $ cat /proc/modules

You can (as root) add new modules to your kernel with the insmod and rmmod commands:

$ insmod modname.ko $ rmmod modname

These two utilities provide a simple, clean way to insert or remove modules from the kernel. If you need more advanced control over the insertion, removal and alteration of modules in the kernel, use the more fully-featured modprobe utility instead.

Writing a Basic LKM

Linux kernel modules are compiled objects written in C

See Also

The Linux Kernel Module Programming Guide- an outdated but solid tutorial covering many of the concepts that will help you to understand the linux kernel.