The big difference is :
You can't play soccer on grass or beach soccer with an official ball designed for futsal played on hard surface .
There is a futsal committee in FIFA not indoor soccer. Also there is the original Futsal International Federation - FIFUSA or Federation Futebol de Salao that gave the origem of Futsal. I voted on behalf of futsal in Canada do move to FIFA along with other Fifusa's futsal members to become part of the FIFA Futsal history.