Keras and tf.keras is an API specification, that is in use for Deep learning and artificial neural networks.
Keras is an API specification that is used to better describes how a Deep Learning framework ought to implement certain parts that are related to the deep learning model and its training. This API specification supports multiple backends like TensorFlow, Theano, Caffe, etc.
Whereas tf.keras is an API specification that is better explained as the TensorFlow implementation of the Keras API. What this API specification did is that it added up many TensorFlow features support to the framework, and eventually the user could get the best of both the worlds, that is TensorFlow and Keras through a single API specification.
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