Keras is an open-source high-level neural networks API written in Python. It acts as an interface for building deep learning models on top of backend libraries like TensorFlow, Theano, or Microsoft Cognitive Toolkit. Keras is known for its simplicity and user-friendly design, allowing developers to build complex neural networks with just a few lines of code. It supports convolutional networks, recurrent networks, and various other types of deep learning models. Keras has become one of the most popular frameworks for deep learning, particularly for rapid prototyping and research.