I have a few questions about Ethereum and tokens.

1. Do i need to save the private key and json file if i use MEW wallet with Ledger nano s?

2. Tokens that run on Ethereum blockchain , how does that work?

2a. If i send for example Modum, does Modum use the same blockchain as Ethereum or some kind of sidechain?

3. What is really ERC20 tokens?

4. Is gas limit the same as transcation fee is for Bitcoin?