How does CSS change the border and css border when you move the cursor over the image?

How does CSS change the border and css border when you move the cursor over the image?

How does CSS change the border when you place the cursor over the image:
Suggestion: writing code as much as possible can effectively improve learning efficiency and depth.
Many Web pages have this effect. When you place the cursor over an image link, the border of the image changes.
The following describes how to achieve this effect.
The instance code is as follows:

<! DOCTYPE html>  
The above Code uses the hyperlink pseudo-Class a: hover to change the border when the mouse is placed on the image.
 
The original address is: http://www.softwhy.com/forum.php? Mod = viewthread & tid = 4658.
 
More: http://www.softwhy.com/divcss/