TensorFlow is a software framework for building and deploying machine learning models.
It was developed by Google for their personal use, but they ended up deciding to make it public in late 2015. The first stable version came in 2017.
TensorFlow is open source. This means it is released under a license which allows users to do basically whatever they want with the software.
TensorFlow is low-level and can take quite a few lines of code to build stuff. Keras is a popular, high-level TensorFlow wrapper for building neural networks with much fewer lines of code.
Keras simplifies the process of coding common tasks in TensorFlow, making it much easier and quicker. Read a little bit more about Keras here.