What it is: A Singleton ensures that a class only ever has one instance throughout your application, and provides a global point of access to it. 

What it is: The Factory pattern creates objects without exposing the creation logic to the caller. Instead of using new directly, you ask a "factory" to give you the object you need. 

Caller just does:

What it is: The Observer pattern defines a one-to-many relationship where when one object (the Subject) changes state, all its dependents (Observers) are automatically notified and updated.

Think of it like a YouTube subscription — one channel (Subject), many subscribers (Observers). When the channel posts (state change), all subscribers get notified automatically.