The spot ETFs hold actual bitcoin. Non-income producing bitcoin. 

The future ETFs don’t hold actual bitcoin. Those would typically hold US Treasury Bils plus bitcoin futures contracts that trade on the CME (Chicago Mercantile Exchange). 

Spot ETFs generate no dividends. 
Future ETFs generate dividends. 

Both types are registered with the SEC (in the US; have management fees (expenses) and are tradable in brokerage / IRA / retirement plan accounts. 

Obviously do your own research. Can go deeper if you want.