It's been a while since I worked on some Java but I believe it has something to do with one being a type (like int) and the other being an instance of the class

It's been a while since I worked on some Java but I believe it has something to do with one being a type (like int) and the other being an instance of the class String. I was always taught "just use String" but I'd love to know the real difference. Google is not proving fruitful for this question.