It is a disease I live with everyday and one I never wish upon my worst enemy!
Seriously though, it is as gimel says an auto-immune disease. The thyroid goes along merrily, and the body thinks it out to move out, and so starts attacking it. Everyone has different and varying symptoms of it. Many people end up having their thyroids removed if after many years the thyroid medication doesn't work. That is not the case for everyone of course. Many people, my sister included, are perfectly ok on medication for many years.
It is a disease in which the autoimmune system produces antibodies that attack the thyroid glands continuously until the glands are destroyed. This can occur over a number of years. Along the way the capacity of the thyroid glands gradually declines, requiring increasing amounts of replacement thyroid meds to offset the loss of natural production. It is the most common cause for hypothyroidism.
If you want more detail, here is a good link.
http://en.wikipedia.org/wiki/Hashimoto%27s_thyroiditis