Sometimes I get a book that I think is YA, only to discover partway through that it's actually a middle grade (MG) book.  For me, that's not a bad thing, but I know others who prefer one category over the other.  Did you know there was a difference?  It's more than just the age of the protagonist and where it's shelved at the library.  Here are some of the basics of each category:

• Books are primarily for kids ages 9-13
• Main character usually falls within that age range as well
• Books are often shorter and simpler
• The issues/darkness is generally dealt with secondhand (MC's mom is an alcoholic, not the MC himself)
• The book is often written to the target audience, helping them figure out right and wrong and who they are
• More outwardly focused (plot changes are more important than how the protagonist changes - though how protagonist changes still important)
• Think Middle School (first time not being a child, learning who you are)
• Generally have happy endings

• Books are primarily for kids ages 14-18
• Main character usually falls within that age range as well
• Books are often longer and more complicated
• The issues/darkness is generally dealt with firsthand (MC is a teenage alcoholic)
• The book is often written from the vantage point of the target audience, helping them figure out how the world works and how they fit in
• More inwardly focused (how the protagonist changes is more important than the plot - though plot still important)
• Think High School (Becoming more individualized, beginning to look outside self)
• Endings can be happy or not

Of course, there are books in each category that don't follow these guidelines exactly, but most do.  If you want to read more on the differences between the two, there are some nice articles on the topic here, here, here, here, and here.