CRASH COURSE!
Laminitis literally means inflammation of the laminae. The laminae is the structure linking the coffin bone to the wall. This connection can be compared to a Velcro Band, it is secure but can easily be taken apart.
So now, a problem arises, metabolic or mechanic, and the lamellar connections break.

Here is an illustration of what happens in the foot when the lamellar tissues disconnect.
The Green line highlights where the wall is.
The Red line show the dorsal wall of the coffin bone.
The Blue line shows where the dorsal should be if the horse wasn’t laminitic – parallel to the hoof wall.
The space between the blue and red lines would be filled up with lamellar wedge.
The pink arrow then highlights the rotation of the coffin bone happening.
The Yellow lines should align. The Yellow arrow shows how the phalanx is sinking into the foot.
So, what it means is that the lamellar connective tissues are no longer able to perform as they should. Hence they break, the coffin bone separates from the hoof wall and the bone column sinks into the foot.
THE SIGNS
First of all, you’ll notice that your horse is unwell (very unwell depending on the grade of lami.). You’ll notice increased sensitivity, digital pulse and legs temperature. It usually starts in the front legs and also reaches the back legs for the worst cases. It can happen in a matter of hours, depending on the horse’s condition and the environment he/she’s in.
In very severe cases, you’ll observe the founder stance, which is pictured at the top of the artcle.
Other signs you can read on the feet:

Here is one of the signs that can be read in the foot, telling you the horse can have a sinking episode.
The blue line, highlighting the collateral groove, should be straight.
But if the coffin bone drops, you can see a step forming in the collateral grooves.
That’s P3 pushing down on the sole.
Because the coffin is attached to the coronary band, when it sinks into the foot, it also pulls P3 down.
The blue line shows where the coronary band is located.
Whereas the green line shows where it should be.

Traces of blood in the white line can just be bruising or it can be sign of inflamation.
Later on, you could find lamellar wedges: the white line is stretched and you can see the how the laminae connections tried their best to maintain the hoof wall attached to the coffin bone.
The 1st picture shows an extreme case. If we had X-rays for this horse, surely we’d see a completely remodeled coffin bone.
CONCLUSION
As you see from the top picture, Laminitis is not only a “fat boy” problem. Pay close attention to the signs and do everything you can to prevent your horse ever knowing this situation, or ever going through it again, by:
– Cutting cereals from the feed
– Providing as much free movement & excercise as possible
– Carefully selecting your horse’s forage & access to forage: 24/7 access, hay better than green grass
– Providing the necessary supplements
– Checking your horse for metabolic issues (Cushings, IR, EMS, PSSM)