[Asterisk-Users] Documentation link,

[Wilson Pickett]

> the recommended documentation source to study and understand dial plans.

Here's a few

http://www.voip-info.org/wiki-Asterisk+Dialplan+Introduction
http://www.voip-info.org/wiki-Asterisk+config+extensions.conf
http://www.voip-info.org/wiki-Asterisk+Extension+Matching
http://www.voip-info.org/wiki-Asterisk+config+extensions.conf+sorting

You could also read the docs available here:
http://www.asteriskdocs.org