Futsal explained

There are, of course, numerous forms of five-a-side or indoor football being played throughout Europe at the present time, and the rules vary considerably from one form to another.

In Futsal, the following rules are of particular note:
1) Each team comprises 12 players. Five players may take the field at any one time, usually four outfield players and a goalkeeper, although the use of a goalkeeper is not obligatory. 

2) “Flying” substitutions are used. This means that the referee does not need to stop play before a substitution can be made. In some cases, four or even five players are substituted at once.

3) Two referees (one main referee and one assistant referee) are used. These referees supervise play by running along the touchline. If there is any doubt between the two referees about a decision, the main referee’s decision is final. In most international matches, an official timekeeper is also used. All the referees used for international Futsal matches are specialized Futsal referees whose names appear on the FIFA List of Futsal Referees.

4) When the ball leaves the pitch along one of the touchlines, it is kicked in, rather than thrown in. When the ball goes out of play behind the goal line after being touched last by one of the attacking team, the goalkeeper restarts play with a throw-in, rather than a goal kick.

5) The matches are played over two halves (2 x 20 minutes of actual time). Every time there is any stoppage in play (corner, kick-in, free-kick, penalty, goal kick, injury, etc.), the clock is stopped (as in ice hockey, basketball, etc.). A system of accumulated fouls is applied. Once a team has committed more than five fouls in one half of play, the opposing team is awarded a direct free kick. The offending team may not use a defensive wall at such free kicks.