Laminitis is a condition where the fingerlike projections on the inside of the hoof wall and the ones attached to the bones of the foot become inflammed...this makes their hold on each other (which what holds the bones in the foot in the proper position) weaker and can allow the bones of the foot to actually sink down or to rotate so that the pointed "toe" of the coffin bone spins towards the soal of the foot. Founder is a commonly used term for the chronic painful condition when the bones have shifted their position.
Laminitis can be triggered by tons of things--toxins from a bacterial infection anywhere in the body, colic, overload of carbs (getting into the feed bag), too much green grass all of a sudden, etc. It's commonly seen when a horse has a metabolic condition called Insulin Resistance--this is when the green grass or even normal amounts of carbs becomes a major causative factors in bouts of laminitis.
Here are a couple of really good informative links for you:
http://www.thehorse.com/ViewArticle....2&kw=laminitis http://www.thehorse.com/ViewArticle....3&kw=laminitis 
