An abscess is basically an infection of the lamina.  Bacteria gets in, causes an infection and then white blood cells attack the infection.  Normally this would cause swelling but because this is in the hoof and the hoof cannot expand, pressure builds and results in pretty severe pain.  The purulent fluid (pus) that is the source of the pressure (product of the body fighting the infection) needs to find a way out.  Generally it takes the easiest path which could either be a hole in the hoof (drilled by either a vet or a farrier) or through the coronary band.

Causes:
Introduction of bacteria to the lamina resulting in infection.  This can happen a number of ways:
-Pierced sole
-Wall cracks
-Bruised sole
-Unsanitary living conditions (softens the sole which allows bacteria in)
-Infrequently cleaned hooves (again, softens the sole which can allow bacteria in)

Symptoms:
-Horse appears lame, doesn’t want to put any weight on the foot (usually a sudden and extreme lameness)
-Increased digital pulse in the hoof
-Swelling of lower leg
-Decreased ground contact with heels

Treatment:
-Your vet or farrier will hoof test to try to pinpoint the location of the abscess, they will then cut a small hole for fluid to drain
-Be sure to keep the drainage hole clean so no new bacteria is introduced
-If unable to pinpoint the location, your vet/farrier might recommend soaking the hoof or poulticing the coronary band to soften it to provide an easier path for the pus to escape through
-To increase blood circulation and promote healing, your vet/farrier may recommend hand walking your horse

Prevention:
-Regularly pick your horses hooves
-Keep your horse in a clean, dry area
-Use a qualified and competent farrier.

If your horse has had laminitis in the past they may be prone to abscesses so be sure to keep an eye out! 

Note: I found an article regarding soaking a horses hoof to help bring out the abscess and according to it repeated soaking, over several days, can result in the hoof wall to loosing integrity causing it to expand or bend outward.  Take a look at this article and remember to always consult your vet or farrier!  www.equipodiatry.com/footsoak.htm