There are usually two players in the position of fullback (wide defender) on a soccer team.
These players are an essential part of the defense on a team. They run a great deal of the defense on the outside and prevent many dangerous crosses, shots, and drives on the goal.
They need to be able to tackle well, control the ball, pass well, and also be able to help with offense.
The fullback position is similar to that of the winger. They both run the outside of the field (shown below as the marked area).
However, the fullback runs more of the sideline defense and is more of a supportive player on the offense, while the winger runs more of the sideline offense and is more of a supportive player on the defense.
When on the defense, the fullback tries to keep a line with the stoppers, as well as keeping a little way behind the winger for more defensive support.
This structure in defense provides a set which is hard for opposing offense to cut through.
Yet, it is good to keep in mind that the structure is not everything and can be broken if you have a rogue offensive player that you need to pick up.
Sometimes the opposing team will put an offensive player toward the edge of the field and about in line with the last defender, so that the offensive player may just run past after a pass is made for an easy goal.
If you are the last defender when this happens, you merely step forward enough that the offensive player is behind you before the pass is made and he/she will be called offsides if the opposition continues with the strategy.
On the offense the fullback will come forward to form a line with the center midfielders and continue to stay a short way behind the winger (who will be lining up with the strikers).
His job on the offense is to be a supportive player for the other offensive players.
He is sort of a dropping point to keep control of the ball, send crosses for the strikers, and assist the winger in beating opposing defensive players (this sometimes means crossing to the front of the winger and temporarily switching places).
You might play with people who will also call the stopper a fullback. Don't be confused, this is very common (especially in America).
Usually these people will refer to all four defense as fullbacks. Sometimes they will have the wide defenders stay back when the offense go forward so that the defense will always be ready in case of an opposing breakthrough.
However, I have always found that most of the time the offense really needs the help (even when you have a striker who has incredible ball control), and the fullback just needs to learn to drop back to defense quickly.