There is actually no difference between them, but there are several cases when you have to use the one of them, instead of the other. To me, the choice between these two words comes naturally, so I don't really know how to explain that to you, but I can give you a lot of examples of the usage, as much as you need to become able to understand the difference.
When it is needed to use the word "возвратить":
- Я всегда возвращаю долги.
- Хороший друг всегда возвращает долги.
When it is needed to use the word "вернуть":
- Верни мне книгу, которую ты у меня вчера взял, пожалуйста.
- Саша, я обязательно верну тебе деньги, но не сейчас.
You don't have in English the Perfective Aspect, but it's like "to listen" (возвратить) and "to hear" (вернуть) if you know what I mean...