How plugins in WordPress work?

Hello!

I am new at WordPress, recently changed from SMF (forum system) to WP and i am trying to understand how the plugins work here.

Today i installed an addon to add some social share buttons on my blog. Sadly something happened with this plugin because i had some text warning (like the code was broken) on the header when my site load.

Nothing yo say, i deactivate the plugin and delete it. Everything was working as normal aftar that.

So, after this i was wondering if the addon added some code into my WP core or something like that or, it is simple loaded from the files on the plugin folder and does not even touch any WP file.

Like i said i am very new at WP so please try to explain how a plugin works in WP.

When i delete a plugin, it completly (100%) remove every part of it of WP, or something is left behind?

Plugins edit the WP core files? or simply loads from the folder?

Thanx in advance for the help!