ILSVRC2012_validation_ground_truth.txt contains class labels for the ImageNet val set (the one that usually gets downloaded).
It uses the mapping in ILSVRC2012_mapping.txt.
I use imagenet_class_index.json as image classes to interop with the robustness library (https://github.com/MadryLab/robustness).
words.txt contains mapping from wordnet IDs to text descriptions.
wordnet.is_a.txt encodes the wordnet hierarchy.