Goalkeeper ("GK")There is always just one Goalkeeper per team. The Goalie's job is to defend his team's goal and he usually stays close to his goal. You can identify the Goalie because he wears a different colored shirt/uniform. As long as he is in the "Penalty Box" he can use his hands to pick up the ball, but if he comes out of the Penalty Box he cannot use his hands.
Defenders- also called Fullbacks ("FB")Fullbacks play closest to their goal (which is the goal their Goalie defends). Along with the Goalie, they have the primary job of stopping the opponents from scoring. However, to a degree, every player should be a "defender" when the opponent has the ball. There are usually 3 to 5 fullbacks on the field per a team.
Midfielders ("MF")The Midfielders play between the Fullbacks and the Forwards. They are often in the "Middle Third" of the field. There can be "Offensive Midfielders" who play closer to the Forwards and "Defensive Midfielders" who play closer to the Fullbacks. Midfielders help connect plays between their own fullbacks and forwards. There can be 2-5 midfielders on the field per a team.
Forwards ("F")Forwards play closest to the opponent's goal, which is the goal guarded by the opposing Goalie. The Forwards are the primary goal scorers, although Midfielders can sometimes score as well. There can be 1-3 forwards on the field per a team.
|As long as you have 11 players on the field (which includes 1 goalie and 10 field players) you can put any combination of fullbacks, midfielders, and forwards on the field.|
|4 defenders, 4 midfielders, 2 forwards (4-4-2)
||4 defenders, 3 midfielders, 3 forwards (4-3-3)|
|5 defenders, 3 midfielders, 2 forwards (5-3-2)
||4 defenders, 5 midfielders, 1 forward (4-5-1)|