Troops, either friendly or unfriendly (ie, yours or the enemy's), block line of sight. Because of this, normally only models in the front rank of a unit are able to shoot, as those behind will not be able to see past their friends.
This does not apply if a target behind normal-sized models (such as Men or Dwarfs) is defined as a large target in its special rules. Large targets can draw a line of sight over interposing models that are not large targets themselves. Orcs or Goblins can't block line of sight to a Giant (large target), for example! This works the other way round as well, for example, a Dragon (large target) can be seen and shot at by enemy models over interposing friendly models that are not large targets, even by models in the back ranks of a unit of troops armed with missile weapons.
Note that this does not allow large targets to charge through any interposing models, which would object to being trampled by the huge creature.