what's the difference between plug-in, hook and hack-in?

[Dismounted]

Do not edit the XML file directly. Create and edit plugins/templates in the Admin CP, and when you're done, export the XML. This prevents structural errors.

The ".=" you are talking about is the concatenation operator (I dare you to say it!) in PHP. It is used to append content onto the end of a variable, etc.

As for eval(), anything in between its parentheses is executed as PHP code. See the PHP Manual.