The main purpose of using these libraries are to store information and retrieving them, but the main difference will be in format for data that you are going to store.
Document Library: These libraries are mainly used to store documents in the form of word, text, PowerPoint, excel sheets, CSV and so on and providing access to different people to use them and modify.
Form Library: It is mainly used to store InfoPath forms where usually they are in XML format and the group are the user who has access will have a chance to work on it from Form Library.
Note: Respective library will have their own inbuilt tools for developing respective things by using tools.
