I came across this well-written article on Linux Kernel Modules (LKM) by Tim Jones. Think of LKMs as loosely equivalents of DLLs for the Linux Kernel.
I highly recommend this article for all interested in improving their understanding the way the Linux OS works. Tim has also authored similar "Anatomy of .." articles on topics like Linux file system, networking stack, the kernel and so on. Here's the complete list of Tim's articles.
Read: Anatomy of Linux loadable kernel modules - a 2.6 perspective (found on doggdot.us)
See also: Other articles on Linux/UNIX