A common mistake among authors who write for the younger set is to refer to young adult and middle grade as genres. That's not the case - middle grade and young adult refers to the age range of the target audience, also called a category.
That's where things stop being simple.
Middle grade and young adult have overlapping areas, and what of the term upper middle-grade? What kind of content is acceptable in middle grade? Is it only the age of the protagonist that determines whether the category is MG or YA?
These are all great questions, and so I put together a brief podcast episode to address exactly this issue. Enjoy!