Middle grade is a category for readers aged between 8 and 12; the conflicts the characters go through reflect those experienced by children in that age group. Stories often have to do with the characters developing relationships with other people and coming to terms with the world around them. There is usually a trusted adult figure in these books.
Middle grade books tend to be shorter than young adult ones. Also, young adult books usually explore deeper ethical and societal problems, and sometimes contain explicit violence. Another distinguishing characteristic is that young adult books normally contain a romantic plot, while middle grade books do not.
There are many middle grade books that were not considered middle grade at the time of their publication, but are categorized so now because of age of the protagonist they portray. Notable examples are fantasy classics such as The Golden Compass and The Lion, the Witch, and the Wardrobe.
If you write middle grade, explore the subgenre to connect with other like-minded writers, workshop, and publish your book.