While building deep neural networks in TensorFlow is far easier than doing it from scratch, TensorFlow is still a very low-level API. Keras is a high-level API that allows us to use TensorFlow (or alternatively Theano or Microsoft's CNTK) to rapidly build deep learning networks. 

Models built in Keras and TensorFlow are portable and can be trained or served in native TensorFlow as well. Models constructed in TensorFlow can be loaded into Keras and used there as well.