Heidegger thought that language is a key to understanding human existence so these subtle differences in definitions are significant, and sometimes difficult.
"Sein," in Heideggerian terms often equates with "being as such" while Seiende is defined as "whatever it is." Sein, then is the very general notion of being, what it means to be - not from a human's perception, but simply being or existing: very general. Seiende refers to things, entities, objects in the world. Seiende is also defined as "beingness" as it refers to the beingness of the things in the world.
Given that Heidegger was interested in human beings and their questioning of Being, we have to focus on that relationship and relationships in general. So, Sein is being in general. Seiende is the beingness of things in the world, and therefore refers to the relationship between the things of the world. So, Sein is being but Seiende is a bit more particular, referring to the relationships between things. (Sein is therefore understood as "existing" while Seiende calls to mind a more ecological framework of the beingness of beings.)
Note, that Dasein refers more to the human apprehension/relationship to the world. Sein and Seiende do not involve a consciousness's conscious relation or interaction with the world. This is Dasein, and what makes human being-in-the-world unique.