They are all basket of assets you are trading in bulk:
Mutual funds are actively managed, meaning their fees are rather high.
Index Funds are just tracking a segment of the market. Low fees but are only priced once a day. It's the preferred passive investment strategy.
ETF are like a combination of the two. They are more versatile & usually track industries, commodities etc. ETFs are more akin to equities than to mutual funds. They can be bought like individual stocks.