What is the difference between letter, communication and journal paper?
In physics, each category has a different length limit.  Often "paper" has no limit.  Typically the shorter the length limit, the more prestigious it is and the tougher the acceptance criteria.  The subject matter covered by the journal is the same for all categories.  Some journals also have a "review" category which includes papers which are not original research.  Often a review is by invitation only.