PyTorch is an open-source machine learning library developed by Facebook's AI Research lab. It is widely used for applications in deep learning and artificial intelligence. PyTorch provides a flexible and dynamic computational graph, allowing developers to change the network behavior on-the-fly, which makes it easier to debug and experiment with different models. One of the key features of PyTorch is its tensor library, which is similar to NumPy but optimized for GPU acceleration. This enables faster computations, making it suitable for training complex neural networks. PyTorch also supports automatic differentiation, simplifying the process of calculating gradients for optimization.