I think there may be different responses to this and that the best answer one can give would be to discuss with your own teacher and instructor as to how they differentiate between the two. For me, an overview is broad in scope, while a summary might be a bit more detailed in its attempt to give an encompassing view of what happened in the chapter. The overview might acknowledge some elements of the chapter that fit into the larger scope and sequence of the work, in general. The summary of a chapter brings out more individualized nuances that are present within this chapter, and while focused on giving a broader view than actually having to read it, still is able to retain some level of subtleties in the process. I think that many study guides on enotes can display this difference quite well. For example, if we looked at the enotes overview and summaries for many works, we can see that an overview or summary of action is something general, with some specifics present. Yet, if we looked at what is offered for specific chapters, these summaries are much more detailed, oftentimes with analysis based off of the summaries accompanying them.