Flask is a minimal, sync-by-default WSGI framework with no built-in validation or docs, relying on extensions like Marshmallow or Flask-RESTX for those. Django is a batteries-included, traditionally sync/WSGI framework that ships an ORM, admin panel, auth, and migrations together, with Django REST Framework as the standard way to build APIs on it. FastAPI is an async-native ASGI framework focused on type-hint-driven validation and auto-generated docs, without a bundled ORM or admin panel, and its edge is first-class async support for high-concurrency I/O-bound services.

Interview line: reach for Django when you need a full batteries-included app (admin, ORM, auth) fast; Flask for a small, unopinionated app; FastAPI when you're building an API-first service and want async performance plus validation and docs for free. Flask's huge ecosystem of extensions makes it flexible but means you assemble validation and docs yourself. Django's opinionated, all-in-one structure is the best fit for large, full-stack apps that want one standard way to do things. FastAPI trades those bundled features for a smaller, API-focused core built around Python type hints.