A kernel can be monolithic or not. A monolithic kernel is a kernel that contains all the driver code needed to use all the hardware connected to or contained within the system you are configuring. Such a kernel does not need the assistance of modules. So why doesn't everyone just build a monolithic kernel and be done with it? The main reason is that such kernels tend to be rather large and often won't fit on a single floppy disk. If the kernel does not fit on a single floppy disk, it poses problems when creating an emergency boot disk. Another reason is that the implementation of a new improved driver for a certain piece of hardware immediately results in the need for a new kernel. This is not the case with modules as will be explained in the section called “Using kernel modules”.

Most of the time, a kernel image is compressed to save space. There are two types of compressed kerneltypes: zImage and bzImage.

The difference between zImage and bzImage files lies in their different layout and loading algorithms. For zImage the maximum allowed kernelsize is 520Kb (see: Johnson01), bzImage doesn't have this restriction. As a result of this fact and the increasing size of the linux kernel, the bzImage kernel now is the preferred image type.

A kernel version number consists of three parts: major, minor and the patch level, all separated by periods.

The major release is incremented when a major change in the kernel is made.

The minor release is incremented when significant changes and additions are made. Even-numbered minor releases, e.g. 2.4, 2.6, 2.8, are considered stable releases and odd-numbered releases, e.g. 2.1, 2.3 are considered to be in development and are primarily used by kernel developers.

The last part of the kernel version number is the so-called patch level. As errors in the code are corrected (and/or features are added) the patch level is incremented. You should only upgrade your kernel to a higher patch level when your current kernel has a security problem or doesn't function properly or when new functionality has been added to the kernel.

Linux kernel modules are object files (.ko files) produced by the C compiler but not linked into a complete executable. Kernel modules can be loaded into the kernel to add functionality when needed. Most modules are distributed with the kernel and compiled along with it. Every kernel version has its own set of modules.

Modules are stored in a directory hierarchy under /lib/modules/kernel-version, where kernel-version is the string reported by uname -r, such as 2.6.5-15smp. Multiple module hierarchies may be available under /lib/modules if multiple kernels are installed.

Subdirectories that contain modules of a particular type exist beneath the /lib/modules/kernel-version directory. This grouping is convenient for administrators, but also facilitates important functionality to the command modprobe.

Module directories are also referred to as tags in the context of module manipulation commands.