Wikipedia:Don't confuse Trademarks with Copyrights
|This page in a nutshell: Trademarked images are not non-free, unless they are non-free.|
|This essay contains the advice or opinions of one or more Wikipedia contributors. Essays are not Wikipedia policies or guidelines. Some essays represent widespread norms; others only represent minority viewpoints.|
Simply put, trademarks are used to protect use of a name or other identifying mark in relation to a product, service, or any other entity. Copyright exists to let authors have the exclusive right to their creation for a limited time period. While something may be protected by both (say, a logo or something), many logos just aren't protected under copyright laws. While Trademark law kinda fills this void, that does not make it non-free. Internal consensus states that the Definition of Free Cultural Works only deals with copyright related restrictions.
Trademark law is a completely different beast to deal with, but it is out of the scope of what can be defined as free content. Sure, you can use Wikipedia content legally for your own encyclopedia, but you cannot use our logo or specifically use the name Wikipedia to brand it on the front cover, because the name Wikipedia and the Wikipedia logo are registered trademarks.