It is a deep learning library which runs on the top of Tensorflow, theano or Microsoft cognitive toolkit. Tensorflow is not easy to use but Keras (high level API) is more user friendly and easy to use. It was developed to make implementing deep learning models as fast as possible. API is the acronym for Application Programming Interface which allows two applications to talk to each other. You can consider API to a translator between two people who speak different languages. Keras API integrates seamlessly with Tensorflow. Companies like Netflix, Uber, Yelp are using Keras. For more details please note down that Amazon is currently working on developing MXNet backend for Keras.