Microkernels do not provide the core operating system functionality, so this most be loaded by other services also present.  Some of these services do not have to be loaded, and new ones can be written also.  This provides the system with a lot of flexibility and performance.

Other Kinds of Kernels
There are many other types of kernels.  The most common concepts are outlined here.

Monolithic Kernels
Monolithic kernels are the most common and traditional kernel styles.  Many operating systems, such as Microsoft Windows, Linux, and FreeBSD are the most well known of this group.  Monolithic kernels put all their functionality in kernelspace.  The code is tightly compacted, and if one component fails, so does the rest of the system.

Exokernels
Exokernels are the antithesis of monolithic kernels.  They run all system programs in user space, therefore not requiring kernelspace.  Unfortunately exokernels are extremely difficult to make and are for the most part impractical.  Very few exokernel operating systems exist.

The Advantage of a Microkernel
While other kernels will suffice, microkernels represent the cutting edge in system software.  The microkernel is a very robust, secure, adaptable, smaller, and higher-performing system.  While relatively few open-source microkernels have been successful, some of the most popular and functional commercial operating systems use this approach.  The most commonly known of these is Apple's Mac OS X.

Other Microkernels
There are many other microkernels that have been made.  Some are listed here.

GNU Hurd
One of the most well known microkernels, the GNU Hurd project unfortunately never was fully implemented.  However, it is still a full-scale project that can be worked with today.

Mach
Mach is another commonly known microkernel project, and some of Quantix's source code is based on it.  Mach influenced the NeXT OS, on which Mac OS X is based.  There are many other systems it has been developed for, including the Intel x86.