Keras is a high-level neural networks API, written in Python and capable of running on top of TensorFlow, CNTK, or Theano. It was developed with a focus on enabling fast experimentation. Being able to go from idea to result with the least possible delay is key to doing good research. The core data structure of Keras is a model, a way to organize layers. The simplest type of model is the sequential model, a linear stack of layers. For more complex architectures, you should use the Keras functional API, which allows building arbitrary graphs of layers.