Difference between restful api and normal app?

Hello guys
I need to know when i can have json and xml output from an action why i need to use restful api?
I really confused :neutral_face:
Thanks :rose:

RESTful API is a group of actions returning JSON or XML. If you are comfortable with coding these manually, feel free to do so.

Thank you for reply mate.
Actually im new yii user and read about RESTfull api i think its very complicated to understand , for example i am going to add new user by getting information from user and save it in in user table and also i need to process data ( do heavy sql from three or four table ) and then save data in another table i dont know how to use models in RESTful app , in normal way im alright but with RESTful its nightmare

Do it normal way first then then see if it worh it to invest time in REST stuff Yii provides.

Thanks , but i was looking for a way to understand the REST , thanks anyway mate :rose: