REST API doc

Apart from the feature guides, where can I find the complete documentation for all the REST API features available? In some cases, I found different examples, and the URI was different. I am looking for an article either online or that might be shipped with product to access at work place. When we do a GET req for package JSON, it does return with 100s of fields in it, which we dont input when creating package, so trying to understand whats the purpose of each field and how it matters? This would help for us to do better coding and also understanding the product features more better. I have issues with some PUT requests, where as GET and Post works in most of the cases, so before I feel some issue with product or my account settings, I would like to make sure I am doing correct way. Any help would be greatly appreciated, thanks.