Player positions can be divided into four rows or categories. First, there is the goalkeeper. Directly in front of him/her, we have the fullbacks (defenders), followed by the halfbacks (midfielders) and then the forwards (strikers, wingers). Each position is designated to perform a specific task in a specific area of the playing field. Although players perform these tasks in specific areas of the field they are not limited or restricted to these areas. The important thing to remember is that when your team has the ball everyone attacks and when you team doesn't have the ball everyone is defending. Your team's first line of attack could be a goalkeeper and your first line of defense could be a forward.
The amount of players in each one of these rows or categories is called the "system of play" or formation. For example, 4 defenders (fullbacks), 3 midfielders (halfbacks) and 3 forwards would be a 4-3-3 system of play or formation when playing with 11 players (U11 and above). It is important to note that systems of play should be introduced but should not be given high priority with U5-U10 at the expense of basic and fundamental technical skills of passing, receiving, dribbling, heading and shooting.