What is a "little" ?

Hey! So I'm living with my fiancé and we are both on the extremely kinky side. I've always called him daddy ( he's older and is very dominant in our relationship/sex) and he treats me like a princess . I've heard some things about DD/LG but not sure what it is exactly and how does one classify themselves as this? Is there certain things/behaviors you can or cannot do? How does one pick an age to act out and how do they act that out? Very intrested to learn more (: