What happens in our body when we eat gluten?

If you look through the different paleo books and websites there is one fact which is consistent throughout all of them. Don't eat anything which contains gluten. There are differing opinions about diary, fruits and nuts. Some people don't worry about them and just eat them, others try to avoid them. Gluten is the one thing everyone treats equally. It's pure poison for our body and we should stay away from it. So what exactly happens in our body when we eat gluten? What's so bad about it?