The restaurant model.
Every app has a public side and a private side. Knowing which is which tells you where things belong, and where they must never go.
Frontend
The Dining Room
What users see and click. Runs in their browser.
Anyone can inspect it.
Backend
The Kitchen
The rules, the logic, the secrets.
Runs on a server users can't see.
API
The Waiter
Carries requests to the kitchen and brings back answers.
More on Day 32.
Database
The Pantry
Where the data is kept between visits.
More on Day 33.
The dining room is public. Keep the recipes in the kitchen.