Keras is a high level API on top of other open source neural networks like TensorFlow, Microsoft Cognitive Toolkit, Theano etc. It is developed by Microsoft. If you are starting to learn Deep Learning Framework, this is the best library to start with because it is a layer on top of other libraries. Although, there are many other Deep Learning Frameworks available in the market:
If you want to start learning Keras, I find below site quite helpful. Because it contains info crisp, clear and easy to understand. Do comment if you like suggested website:
Below is the Keras Cheat Sheet created by DataCamp which is quite helpful to recall Keras commands or to go through before going for an interview.