?WP_Forum changes any files of my WP?

Hello to everyone

I just created my personal blog and i was thinking a forum was a good idea to promote the comunication in my comuniy so i tryed to insall WP-Forum.

I ran the installation using the new on line installation wizard in WP 2.7 and the result was a lot of code lines on the header of my blog and, of course, the plugin not succesfully installed.

So i did this:

1) I tryed to deactivate the plugin in the admin but didnt work
2) I renamed the plugin folders name and then magically all this code lines and everything dissapears of my header and the forum was normal like before the plugin.
3) I went to my Database and erase every table this plugin created

Ok now it seems the plugin is deactivated and i dont have any table on my database, but i want to be sure, so my question is:

Im used to work with other systems such SMF, in this forum system when u install a plugin this one inserst lines of code in the core files and tempalte files, so when u want to erase a plugin is a little messy cuase u have to erase manually all this lines.

In WordPress im not sure if this happens too, i dont this this becasue:

1) It seems every plugin in WP have their own folder and make any change on this folder, usually are on /plugin directory of WP
2) In the specific case of the plugin WP-Forum u have to create a custom page so u can actually see the forum, this makes me think this plugin doesnt affect any core or tempalte files at all.

Well this is what i think, but, it is true? Plugins doesnt adds code to the core of template files of WP?

Thanx for the answers.