Hey there! These are actually great questions, let me help you out! Going to answer in an order that will help make the next answer more clear :)

4. IPFS stands for the InterPlanetary File System, and is something Pinata has been primarily known for over the past 5 years. It's a distributed and immutable file sharing network that is often used in blockchain applications. It's similar to storage where the file has to be hosted, but the next step is making it available on the peer-to-peer network which is what we specialize in. Outside of Web3 stuff it's not used much as files can only be public, and that's why we built our Files API.
   1 & 2. "Pinning" is the term used for putting a file on IPFS and making it available on the network. Uploading is more standard cloud file storage.
5. Great use case! I would actually recommend making an admin API key and using it server-side for most operations, such as making a group of files public. For uploads we have a method for client side uploads, however we will soon be releasing a more recognizable signed upload url flow.

This is really helpful perspective for us so we can adjust our docs to make these distinctions a bit more clear. Thank you for asking! Please let me know if you have more questions :)