Questions about cold storage

What exactly is cold storage? Is it a wallet not connected to the internet?. I assume then that any mobile wallet is not considered cold storage.

Can a desktop wallet be used for cold storage? The whole point here is to maximize security of the funds.

For example, if I have a desktop wallet, like electrum, installed on a computer that is not connected to the internet, would that be cold storage?

Also, at some point the wallet needs to connect to the internet, how does that work?

How are hardware wallets different to an offline computer with a desktop wallet for example?

Are all hardware wallets considered cold storage?

It would be great to get to know this in more detail, thanks!