What exactly is the difference between Fundamental Rights and Directive Principles?

The key difference is enforceability. Fundamental Rights (Part III) are justiciable, meaning if the State violates them you can directly go to court, including the Supreme Court under Article 32, to get them protected. Directive Principles (Part IV) are non-justiciable, meaning you cannot sue the government for not following them; they are guidelines for making laws and policies to build a welfare state. Fundamental Rights protect individual freedom (equality, freedom of speech, against exploitation). Directive Principles aim at social and economic justice (right to work, free legal aid, protecting the environment). In short, Rights are guarantees you can demand in court, while Directive Principles are moral and political ideals the State should try to achieve over time.