Is there really a difference between these two? Any links to some publication/articles about playtesting? very hard to come by. Thanks!

Usability is somewhat of a science, especially as far as user interfaces are concerned (this is important for games as much as for traditional desktop software). If you are interested in usability testing, I would recommend starting with this paper, which formalizes some of the related concepts.

As far as play testing is concerned, I would argue that such vague concepts as "immersion", "fun", "replay-ability" play an important role - and while some of these can only be viable given a usable interface, they are not related to the interface per se.