A unique feature of the Linux Kernel is its ability to allow injection of code while it is running. This piece of code is called a kernel module.

The traditional approach employed by operating system designers was to add the code to the source code of the OS and build the OS as a single binary that can be booted by the boot-loader.

The kernel module can be inserted and removed at any time during the execution of the kernel giving Linux Kernel developers the flexibility to test their modules before making them a part of the mainline Kernel source code.

The process of building a stand-alone kernel module or integrating the kernel module with the source code of the Linux Kernel is simplified through the makefile of the kernel module. The source code of the kernel module remains the same irrespective of how it is built.

Information on kernel module programming can be obtained from:
http://www.tldp.org/LDP/lkmpg/2.6/html/lkmpg.html