I am quoting this from an answer on another subject.
"Internal Link URLs are links to internal pages within your shop. These can be to any valid URL, but should be written as relative links such as:

index.php?main_page=index&cPath=21

The above Link would take you to the Category for categories_id 21"
Can someone point me to a tutorial, or explain, the interpretation of the above URL.
What is the reference to Category?
I wish to understand and interpret the URL of any page within a site.
How would I link to a particular product page, from another page within the site using a relative link?
Is there a tutorial that explains this, or if it is in THE BOOK where exactly?