Pretty much what Lian said. It really isn't too bad if you catch it earlier. I use Thrush Buster, its like $20 and you can get it at pretty much any store for equestrians. Thrush really only occurs if they are standing in wet areas. If you are worried about your horse getting it, then just use thrush buster once a week or so and when the ground is very wet.
Thrush is an unpleasant infection of the horse's frog, which is predisposed by moist, damp, dirty ground or stable conditions. Thrush is an infection of the central and lateral sulcus (clefts) of the frog of the horse's foot, most often involving bacterial and occasionally fungal infection. I believe there is a product called thrush buster that can help/prevent it.
Hope this helps