As mentioned before, units are often groups of warriors operating together. They fight in a loose formation with gaps between each model. This gives the individual troopers freedom to move quicker over difficult terrain, and enables them to take advantage of cover.
When you are moving a unit the individual models in it can move by any amount up to their maximum Movement distance. Remember that units have to stick together otherwise individual troopers or vehicles become scattered as the squad loses its cohesion as a fighting unit.
Once a unit has finished moving, the models in it must form an imaginary chain where the distance between one model and the next is no more than 2".
During the course of a game it's possible a unit will get broken up, usually because it takes casualties or individual models are engaged in close combat. When this happens the following rule applies:
If a unit is broken up for any other reason the models in it must be moved to restore coherency in the next movement phase. If they can not do so then the unit may not shoot or launch an assault until it is in a coherent formation again.
Models engaged in close combat are not bound by the above rule. Once combat has finished the unit must form up again.
Turn and Facing
As you move models in a unit they can turn by any amount without any penalty to the distance they are able to cover. Models are turned to face their targets in the shooting phase so don't worry about which way they are pointing at the end of their movement phase (although dramatically facing off against their foes is traditional).
Diagram: Unit Coherency — three example squad layouts. In the first two (a loose cluster and a line formation), every model is within 2" of the next model in a continuous chain, so both are marked correct. In the third example, two models on the right are 4" away from the rest of the squad on the left, breaking the unit's coherency, since the gap between adjacent models in the chain must not exceed 2".