ETFs are just investment funds you can buy and sell in the stock market, with their own symbol and everything just like any other stocks.
Index (passive) funds track an index, instead of trying to outperform the market.
You can have passive funds that are not ETFs (do not appear as a symbol in the stock market, have to go through a bank or whatever to buy shares), and active funds that are ETFs (actively managed).
Presumably any ETF is tracking something, that you could define to be an "index" in a degenerate sense (even if it's the "companies Buffet thinks are undervalued index", or the "solid gold bars index"). Or is there a technical definition of index?
Index (passive) funds track an index, instead of trying to outperform the market.
You can have passive funds that are not ETFs (do not appear as a symbol in the stock market, have to go through a bank or whatever to buy shares), and active funds that are ETFs (actively managed).