Actually, in most web applications, these three terms are used like this:

🧩 1. Frontend

• This is what users see and interact with — like a website or mobile app interface.

• Built using HTML, CSS, JavaScript, or frameworks like React, Vue, or Angular.

• It sends requests (e.g., “Get user data”, “Log in”, “Upload file”) to the backend.

🖥️ 2. Backend (Server)

• This is your FastAPI application (or Django, Flask, Node.js, etc.).

• It sits between the frontend and the database.

• The backend:

  • Receives API requests from frontend

  • Processes data or applies logic

  • Interacts with the database or external APIs

  • Sends the result back to the frontend

💬 Example:

React App (Frontend) ➜ sends request ➜ FastAPI (Backend)
FastAPI ➜ processes ➜ sends JSON response ➜ React App displays it

🗄️ 3. Database

• Stores all your data: users, orders, medicines, files, etc.

• Backend communicates with it directly (frontend never touches it).

💬 Example:

Frontend ➜ Backend ➜ Database

🌐 4. Where CORS Comes In

When your frontend (say, http://localhost:3000) tries to call your backend API (http://127.0.0.1:8000),
the browser blocks it due to different “origins.”

So your FastAPI server adds this:

app.add_middleware(CORSMiddleware, allow_origins=["*"], ...)

✅ This tells the browser:

“Hey, it’s okay! Let this frontend access me.”

💡 In Simple Terms