The overal value of a player is defined by the silver stars you can often see next to your player. These stars are calculated based on the trainable skills, speed, power and position experience.
You can use this value as a general indication of the strength of your player. It's not guaranteed though that the player with the most stars will always perform best. How well a player performs also depends on his role in the team and on the strengths and weaknesses of the opponent.
For each type of player there is a different formula. In the formula for defenders, dueling and defender experience have the highest value, for keepers blocking and keeper experience etc.
A player is called a "defender" if his value as defender is higher than his values on other positions.
If, for example, you want to retrain a young defender as a midfielder you could do this by letting him play as midfielder and training mostly passing. This will raise his midfielders value the most, so eventually the player will be called a midfielder.
Youth players also have grey stars. These grey stars represent the value of the player if he would get the maximum youth bonus in a match.
The actual value of your player in a match can be anything between the silver star value and the grey star value. If you hold your mouse over the stars you'll see the minimum and the maximum value.