Smalltalk
In plain terms
A pioneering, purely object-oriented language and environment.
Going deeper
In Smalltalk everything is an object and computation happens by sending messages between them, all in a live, image-based environment. It deeply shaped modern OOP and GUIs.
Under the hood
Its live image (the whole running system is saved/modified) and message-passing model influenced Objective-C, Ruby, and IDEs.
Examples
Historical OOP research, Pharo, GUI/IDE concepts, some enterprise systems.
Think of it like...
Like a living workshop where every object is alive and you reshape the world while it runs.