Keras is a high-level API running on top of TensorFlow (and other libraries). Keras is preferred over pure TensorFlow since it is much easier to quickly get something up and running. Keras also contains pre-trained ConvNet models, for example VGG16 and VGG19. Training models on the full ImageNet dataset is extremely time consuming, so getting access to a pre-trained model can be very useful.
Keras is a very popular deep learning framework and there are lots of good guides you can find to get you started!