You've got some pretty good explanations so far. Most of other posts were very detailed. I've coached youth soccer, so I tend to start at begining and keep it simple. You probably already know this stuff, but it can't hurt. I'll also throw in some miscellaneous stuff.
One cool thing about soccer is every position uses the same skills. You need to trap (aka settle or control) the ball wherever you play. You need to tackle (steal the ball). You to be able to dribble. You need to be able to kick the ball obviously. The three basic positions place emphasis on different skills, but everybody uses all of them.
Most important skill at EVERY position is trapping the ball. You must be able to control the ball at every position. You must trap the ball first - before you can dribble, pass, or shoot. Watch the difference between good/bad teams, you'll immediately see it in their trapping. You could be the greatest shooter in the world, but if you can't control the ball you'll never score during liveplay.
Forward/strikers - they are offensive players. You're there to either score goals or assist on somebody else's goals. They do some defending, like forechecking in hockey. They are mostly trying to steal ball to start "fastbreak" opportunity for team. Typical range for forward is on offensive side of field (60 yds or so). Forwards rarely come back further than midfield. Forwards must be able to beat defender one-on-one. Can do this by making/receiving good pass, finding open space, or dribbling.
fullbacks/defensive backs - they are defensive players. Job to stop the other team's offense. This position requires the most teamwork and communication. You hand off offensive players to one another, you work offsides traps, call out plays, etc. Typical range is defensive half of field, but might spring forward if they have the ball. Kind of like cornerback on an island, they must be able to stop folks one-on-one. Best play is to take the ball away, second is slow down the ball, third is make ball go out wide.
midfielders - they are both offensive and defensive players. When team is on offense, they move up with attack. Likewise, when team is on defense, they are back to help defend. These are smartest players on field. Have to feel flow of game. Their range is penalty box to penalty box. Gotta have lungs to play here. Since they play offense and defense, they use same techniques as forwards and defenders.
[This message has been edited by birdman (edited 6/5/2006 8:24p).]