Keras is a high-level deep learning API designed for human-centered, intuitive model building. Originally created as an independent library and later integrated into TensorFlow, Keras enables fast experimentation with neural networks through its Sequential, Functional, and Subclassing APIs. It abstracts the complexity of backend tensor operations while providing full control for advanced use cases, making it the go-to framework for both beginners and production environments. The key insight: Keras treats models as composable graphs of layers—once you understand this mental model, building anything from simple classifiers to multi-input transformer architectures becomes remarkably straightforward.