Apache2 Document Root (htdocs) in Ubuntu

OS: Ubuntu 13.04

Apache2 version: Apache/2.2.22 (Ubuntu)

My comments:

I don't know why web develoeprs makes so complicated.  It took me quite a while to figure out the jargons what'Document root', 'htdocs' are in Apache2. 

1. What is 'htdocs'?

According to
here,  htdocs mean that Document Root. It should be specified 

2. What is document root?

It is the default document place where you put the files. According to
here,  we should edit /etc/apache2/sites-available/default to specify what document root is.  You can specify the document root for other sites, simply by copying the default to the same folder and rename it. The document Root is one directive
in these types of files. 

This is because the apache2 in Debian-based OSes is different from in other OS because it wants to allow more "flexibility". 

The document root is NOT written in the httd.conf any more. But it creates CONFUSION!  

I have no idea why they would make thing so unnatural and arbitrary for new comers.