ETFs (exchange-traded funds) are pooled collections of investment funds; it’s a type of security tracking index, traded the same way as individual stocks.
For example, SPY (SPDR S&P500 ETF) is well known and tracks the S&P500 Index.
ETFs are traded on major stock exchanges, like the NYSE and Nasdaq, which have been available in the US since 1993 and in Europe since 1999.
As of March 2019, more than 5,000 ETPs (exchange-traded products) were trading in the US.