"Center of the field" - The center of the field is the area between the 2 goals (as opposed to the "wings" or sides of the field where the sidelines are). The center of the field is where the "Center Fullback", "Center Midfielder" and "Center Forward" mostly play (you may not have Center positions in your "Formation"; if you play 7 or less on the field you may only have "Right" and "Left" positions). If you say "center it", you usually mean to kick the ball to the area in front of the goal.
"Middle of the field" - The middle of the field is the area of the field that includes the "Halfway Line". Many coaches say "go to the middle" when they really mean "go to the center".
"Attacking Third" - The Attacking Third is the third of the field that is closest to your opponent's goal (i.e., the goal the opposing Goalie defends). When your team attacks, you hope your Forwards and possibly your Midfielders get the ball into the Attacking Third. When your opponent is attacking your goal, you will probably not have any players in the Attacking Third, since your Midfielders will probably have dropped back near your "Penalty Box" line to help defend and your Forwards will have dropped back to the Halfway Line or to the inside of the "Center Circle" so they are in position to win "cleared" balls. (This "shifting and sagging" with the ball is very important so you have "support" on offense and defense).
"Middle Third" - The Middle Third is the third of the field that includes the "Halfway Line" and the "Center Circle". When you attack, if your Fullbacks "Push Up" they will come up into the Middle Third. When your opponent is attacking your goal, your Forwards may come back into the Middle Third to the Halfway Line or even to the inside of the Center Circle so they are in position to win "cleared balls". (See "Push Up" in the Dictionary and "Should You Push Up When You Attack?" on the SoccerHelp Home Page).
"Defensive Third" - The Defensive Third is the third of the field that includes your goal (your goal is the one your Goalie defends). Your Fullbacks will spend most of their time in the Defensive Third unless you have a good team and your Fullbacks "Push Up" when you attack. Your Midfielders will probably drop back into your Defensive Third to help out when your goal is under attack, but your Forwards should not come into your Defensive Third, because if they do there won't be anyone in position to win the ball when your Fullbacks or Midfielders "Clear" the ball.
