What is realism and how does is it different from naturalism?or is it the same in some way?
Many critics believe that there is no significant difference between these two; I would agree. The problem is that we can spend so much time talking about "isms" that we fail to enjoy appreciate the actual texts. I don't know what it is about our need/desire to categorize all experiences, but I'm not sure that it serves us well.
If there is a difference, I would suggest that naturalism tends to view humans as more "mechanistic," more controlled by forces that we do not undertand, perhaps more "animal" for "fate driven" that realists to simply select a certain class of experience, usually the lower, and try to portray it as accurately, as "realistically" as possible without any embellishment. Dreiser would be a good example of the first (An American Tragedy, Sister Carrie) and Crane an example of the second (Maggie).
But, as I said, I don't know how understanding this "difference" adds to our enjoyment of the literature.
I pointed you to two sources on enotes that can provide you with much more details and specific information if you wish to go further with this.