33. Explain the following:

1. Keras

This refers to an open-source neural network library written in Python which is designed for enabling fast experimentation with deep neural networks.

2. TensorFlow 

This refers to an open-source software library for dataflow programming which is used for machine learning applications like neural networks.

3. PyTorch 

This refers to an open-source machine learning library for Python which is based on Torch. This is used for applications such as natural language processing.