This article explores what focus stacking is and why you might want to do it.
To cut a long story short, focus stacking is taking several pictures of a subject all with slightly different planes of focus, then combining those in software to form one composite image with more in focus detail than you could achieve with one shot.
The reason for focus stacking is that at high magnifications (above around 5:1) due to the effective aperture increasing with magnification, you need to use lenses with the aperture wide open (low F-stops) to avoid diffraction stealing all your resolution.
Because as magnification increases the DOF decreases, and as the aperture widens (smaller F-stops) the DOF also decreases, you end up with wafer thin DOF at high magnification, meaning stacking is a requirement for really tiny three dimensional subjects.
This article simply gives you some ideas of the equipment which is used for the technique of high magnification stacking.
Addresses the two main questions when thinking about putting together a focus stacking mechanical rig;
This article lists the different types of lenses which can be used for for this kind of work and talks about the differences;
This article points out some common problems you might experience and how to avoid them or lessen their effects on your output.
The pros and cons of the different pieces of software which can be used for focus stacking.
This page walks you through the steps to processing a stack of images in three different software packages - Zerene, CombineZP and Tufuse pro.
The article includes a link to a zip file containing an example stack of input images available if you want to try out the software but have not yet made any stacks.
Some other pages on the internet which will be useful if you want to try out Focus stacking.