A full back, that is, a left or right sided defender playing to the side of the central defenders should be reasonably quick on their feet with a good tackling ability and ball control. Their function is to not only prevent opposition players crossing the ball to attackers, but also be the first point of attack themselves at the wing. They can often replace traditional midfield "wingers" although they must have good stamina for this.
For any defender position: You need to be aggressive and brave. You can't be afraid to "get in there" and you cant be afraid of the other team. You need to be able to recover quickly if you make a mistake and be able to hold someone off. You have to be mentally and physically quick (you must know where to send the ball once it comes to you and and you have to be ready to sprint anytime). You need to be able to remain calm under pressure like think where players are, who to pass to, think strategies. It doesn't hurt to have a loud voice either, communication is key to defending. You need to be able to communicate with your team mates because they'll be able to help you under pressured conditions. Also, when your off, you should watch players from the other team and learn what they do in one-on-one situations, its very helpful to know which player can do what and how you can out move them.
I disagree with some of this, and rather than taking out what I disagree with, I'm going to continue here. katiewalker
All defenders should be quick thinking and decisive to start with. If you mess up, you have to be able to see that you have and then fix it.
You also have to be strong. Defenders cannot be falling over regularly as that is the surefire way to concede a goal. You don't have to be a body builder, of particularly tall, to fulfill the needs of a central defender, although this helps in selection. You just have to be able to knock the defender away from the ball.
They also needs to have an attacking sense about them, even if they play in the centre. When you have the ball, you need to know where you are going to pass it to help score a goal.
Speed is also important. If you can't outrun, or at least run with the attacker, you have no place on the field. This is particularly so with left or right fullbacks or wingers. Speed is an integral part of their game. Not only are they trying to beat the attacker, they are trying to beat the defenders on the other side of the park.
A back is a defender.
Neelanjan LakshmanPique
Yes, if you are a defender.
defender
Pressure of the defender
Defender...
a defender that is on the right side
John Terry of Chelsea and England.
goalie, defender, midfielder, forward
Today it is the Brazilian Danny Alaves.
Defender or Midfielder
The defender usually depends on the sport. In soccer (football) the defender is the one that protects the goal. I don't know much about football (American Football)