they elgg (sic) team deleted it ? such wonderfully usefull content and they deleted ?? http://docs.elgg.org/wiki/Plugin_development  some | a lot of what you're looking for cannot be 'answered' here in a few short sentences;; you'll need to spend *hours studying to learn what-to and how-to.

I went through a fair amount of documentation and I do start to undestand it a bit more.
I will keep playing with it and ask when i need, thanks.

I also found this guide which help a bit to understand some basic stuff
http://www.solutiongrove.com/docwiki/beginners_elgg_guide