It MUST be kept in a COOL DRY place- not wet/freezing/hot- otherwise it will ferment.
Barley has quite of a bit of digestible energy and is a little more nutritionally balanced that oats (oats are very similar to Barley other than what I just mentioned). It does have a poor phosphorous/calcium ratio and it lacks some vitamin A and D.
Since it is more digestible energy, it doesn't tend to make horses "hot"- or at least the horses who tend to get hot from feeds such as Alfalfa or Oats.
The grains MUST be crushed, rolled, or cooked before feeding as the grains are to hard by themselves. If you end up cooking them make sure they are whole grains, not crushed or rolled.
It should be fed with some other type of grain with fiber- beet pulp, chopped hay, hay pellets/cubes, oats, etc. otherwise it can compact in the horses stomach and cause colic.
Barley should be a paler gold color and not be dusty.