I am told that the following list of "puppy" image URL's are from imagenet.
https://github.com/asharov/cute-animal-detector/blob/master/data/puppy-urls.txt
How do I download another category for e.g. "cats"?
Where can I get the entire list of imagenet categories along with their explanation in csv?
Unfortunately, ImageNet is no longer as easily accessible as it previously was. You now have to create a free account, and then request access to the database using an email address that demonstrates your status as a non-commercial researcher. Following is an excerpt of the announcement posted on March 11, 2021 (does not specifically address the requirements to obtain an account and request access permission but explains some of their reasons for changing the website generally).
We are proud to see ImageNet's wide adoption going beyond what was originally envisioned. However, the decade-old website was burdened by growing download requests. To serve the community better, we have redesigned the website and upgraded its hardware. The new website is simpler; we removed tangential or outdated functions to focus on the core use case—enabling users to download the data, including the full ImageNet dataset and the ImageNet Large Scale Visual Recognition Challenge (ILSVRC).
ORIGINAL ANSWER (LINKS NO LONGER VALID):
You can interactively explore available synsets (categories) in the /explore directory, each synset page has a "Downloads" tab where you can download category image URLs.
Alternatively, you can use the ImageNet API. You can download image URLs for a particular synset using the synset id or wnid. The image URL download link below uses the wnid n02121808 for domestic cat, house cat, Felis domesticus, Felis catus.
http://www.image-net.org/api/text/imagenet.synset.geturls?wnid=n02121808
You can find the wnid for a particular synset using the explore link above (the id for a selected synset will be displayed in the browser address bar).
You can retrieve a list of all available synsets (by id) from the no longer valid:
http://www.image-net.org/api/text/imagenet.synset.obtain_synset_list.
You can retrieve the words associated with any synset id as follows (another cat example).
http://www.image-net.org/api/text/wordnet.synset.getwords?wnid=n02121808