The penalty area is significantly different from football: it is semi-circular in shape, only the goalkeeper is allowed to touch the ball within it, and he or she may or may not be allowed out. Goalkeepers are only allowed to give the ball out to another player through hands. The goalkeeper may only kick the ball if it is in the course of making a save. There are no offside rules. Headers are not allowed. Charging/sliding tackles /trips/kicks are not allowed and result in a direct free kick being awarded, all other offences result in an indirect free kick with the exception of the goalkeeper leaving the penalty.
Five-a-side is commonly played informally, and the rules are therefore flexible and are sometimes decided immediately before play begins, an over head height rule for example should be decided on before the game. For more formal information which could shape the rules you can review the FA’s small sided football rules.
Additionally everyone needs to keep safe, avoid wearing jewellery, wear shin guards and metal studded boots or blades cannot be worn, as it damages the playing surface.