In his own assault phase a player can charge any unit that is within 6" of an enemy unit forward into contact to make an assault. All the models in an assaulting unit make an extra 6" move in exactly the same way as if moving in the movement phase. A unit of warriors that remained stationary in the movement phase in order to fire heavy and/or rapid fire weapons cannot assault as it is assumed to be spending the whole turn shooting.
When a unit makes an assault all the models in the unit must move into base-to-base contact with the enemy if they can do so. The closest models move first and towards the enemy by the most direct route. Once units are fighting at close quarters like this they are said to be 'engaged in close combat'.
Diagram: Assault Move. Models in an assaulting unit each make a move (shown by dashed arrows) directly towards the nearest enemy models, moving into base-to-base contact where possible. The maximum assault move is 6".
Errata — Chapter Approved: Main Rules Clarifications (v2.0): A unit that declares a charge but fails to get any models into contact due to, for example, difficult terrain, does not fight a close combat even if it has models within 2" of the enemy. Note that if the charging models do not make contact they are not moved at all.
Errata — Chapter Approved: Main Rules Clarifications (v2.0): When a unit contains models which can move at different speeds, all models move and assault at the speed of the slowest.