If you're lucky you'll get a couple of kind sentences and some sort of reason why the editor is passing on your tale. Remember that it is not the editor's job to teach you to be a better writer. His only job is finding the best stories for his magazine. You shouldn't expect lengthy explanations on the strengths and weaknesses of your work. However, if you do receive a lengthy rejection, I suggest you listen to all the editor has to say with an open mind.
Some editors can be incredibly helpful with their constructive criticism.
Some can be vicious and degenerate, with their own axes to grind, who enjoy their position of power and like nothing better than to watch newer writers twisting in the violent winds of the small press. Over the course of your struggles you'll probably run into each type. You are charged with remembering that all rejections are merely one person's opinion. If you can take something positive from the comments then do so, if you can't then leave them aside. My suggestion is that you save all rejection letters for the first couple years of your writing life, for one important reason.