I was wondering what exactly are the "bends" experienced by sea divers?

Decompression sick: Decompression sickness (dcs) is also called the bends. When a diver descends in the water- pressure increases. As the diver ascends there is less pressure which allows dissolved gas to form bubbles in body tissues. This can cause a variety of different symptoms ; in some cases can lead to death. This is treated in a recompression chamber.

It is a colloquial: Term to describe decompression illness when a diver ignores mandatory decompression stops upon rising. Gases that have gone into solution (plasma) while under pressure come out of solution too fast, like opening a can of soda. The mechanism of pain generation is too detailed so suffice it to say the gas bubbles are where they shouldn't be. Google boyle's law for the physics.