What is the difference between a snickerdoodle and sugar cookie?

Besides the putting of frosting and sprinkles. I mean do they taste different? different ingredients? I am only asking cuz my girlfriend said they are different but the cafeteria at my college has snickerdoodles that are a plain kinda sweet white colored cookie with sugar sprinkled on top. But I have had Otis Spunkmeyer Sugar cookies and it is a white kinda sweet cookie with sugar on top... What is the difference?