Futsal is a form of wall-less indoor soccer that is played 5 v 5 using a special low-bounce ball. It is the only form of indoor soccer officially recognized by FIFA.

Futsal vs. Soccer vs. Indoor Soccer
Futsal is played the same way as soccer but on a hard floor about the size of a basketball court. Its ball is smaller and less bouncy than a normal size-five soccer ball. Unlike soccer, in futsal there is an accumulated foul count, teams substitute on the fly (like hockey), and the two halves are only 20 minutes each. Unlike typical American indoor soccer, there are no walls and teams play four players (plus a goalie) to a side.

Why futsal?
Because of its smaller playing area and the lack of walls to deflect poorly aimed kicks, futsal is great for developing precise passing. The fast pace of the game forces players to think fast, gives them more opportunities to touch the ball, and leads to higher scoring games. Individual players never have much space or time to dribble, so team shape, rotation, and ball movement are critical. The unique characteristics of the ball make it easy to control and encourage creativity; players’ physical size and strength are not as important as their skill.

History
The name futsal comes from a combination of the Spanish words for “soccer” (fútbol) and “indoor” (sala). The game is said to have originated in Montevideo, Uruguay, in 1930, as a five-a-side version of youth soccer for local YMCAs. Around the same time a similar version was also being played in the streets of Sao Paulo, Brazil. The first international competition was held in 1965, when Paraguay won the South American Cup, and the first Futsal World Championship took place in 1984. The game is now played by over 12 million people in over 100 countries worldwide and touted as one of the keys in the development of soccer greats such as Pele, Zidane, and Ronaldhino. Although popular abroad, the sport has only recently gained a following in the States, futsal has only recently gained a grass-roots interest in the United States.

A longer history of the sport can be found at the USFF website.