The Movement value represents how far a warrior can move whilst going at a fairly rapid rate, allowing him time to aim and shoot a weapon and to observe what is going on around him. If you wish, a model may move much quicker than this - he can run! A running warrior can move at double speed (for example, 8" rather than 4"). Note that running is not the same as charging as it does not allow your model to engage the enemy in close combat.
A model can only run if there are no enemy models within 8" at the start of the turn (fleeing, stunned, knocked down and hidden models do not count). Check this distance after any charges have been declared. If there are any enemies within 8" at the start of the turn, the model will prepare to fight instead and so is unable to run. The running model can move closer than 8" to an enemy as it moves.
Any model that runs loses its chance to shoot during that turn. He is concentrating on running and is not prepared to fight, having sheathed or shouldered his weapons. You should declare that models are running as they move, as this will remind both players that the model is unable to shoot that turn. Running models can cast spells as normal.