Kernel Module

What is a kernel module?
A kernel module is code that can be dynamically loaded into the kernel during runtime. This makes the Linux kernel extensible so that not all functionality has to be compiled into the kernel during the initial creation. From our experience making our own system call, we know that compiling a kernel is a lengthy process and that was a SMALL kernel.
The first purpose of modules you might think of is probably device drivers that allow the kernel to interact with a new type of hardware. However, modules can add a host of other functionality like filesystems, network protocols, cryptographic libraries, etc.