Focus stacking allows you to create a greater depth of field that your lens allows you to capture and is commonly used with macro photography. In this video from Aaron Nace at Phlearn, he walks you through how to shoot and composite focus-stacked images in Photoshop.
How To Do Focus Stacking in Photoshop
[youtube https://www.youtube.com/watch?v=iYA7wb2Vkz8&w=700]
I recently used photo stacking for the first time in Photoshop for a small project I wanted to try (a watch face with my macro lens).
It’s incredibly easy and the results were superb.
I find the video here to be even better than the one I had found on YouTube. His explanations are more clear.