I would like to know "what" is in a cats tail. I've always thought it was a bone but it can't be a bone because you can bend the tail. Is it some sort of cartilage or something?

I believe it's cartilage, I have had a couple cats damage theirs and the vet says it can get infected easily, and sometimes if it's a really bad cut, amputation is needed. Cats tend to not heal very good, and form pus sacks on their cuts, which has to be lanced sometimes. That's why its important to get an antiobiotic in them when they are having these infections under their skin.
I have a black manx, which has 3 vertabraes instead of a full tail, so there is always the exclusions.

This site gives details about a cat's anatomy.
The tail is made of 18-20 supple and articulated vertebrae.

http://www.specificbreed.com/html/skeleton.html

Oh wow! Nice site! Thanks for informing me, I had no idea they had a vertebrae in their tails.