I guessmy girlfriend's mother is a hoarder. When my GF was younger, her and her friend tried to "help" her mom clean her house, and her mom spazzed out when they tried to throw out some of the stuff. After that they swore not to help her mom clean anymore (until after she passes away and the kids have to go through it all). We had a Christmas dinner there this year, and even though her mom had spent weeks cleaning for the dinner, the kitchen (the only room we really went into) still had junk on top of the cabinets, cluttering the China hutch (you couldn't really tell what piece of furniture it was except for guessing based on size and the room it was in), etc. And after that dinner, I understood what my girlfriend was talking about.
I also know another person whose daughter-in-law is a hoarder too. From what my friend reports, her grand-daughter doesn't have her own bedroom because it is completely filled with stuff to the point that you can barely walk into that room. The little girl ends up having to sleep on the sofa instead, and even then usually has to push junk off of it just so she can lie down. She's been reported on and they had to have some group that specializes in cleaning a hoarder's home come and clean it out so that their house wouldn't be condemned. But then after the court hearing, I guess she went back to her hoarding habits.
I sometimes joke that I'm a techno-hoarder -- that I just keep any old electronic gadget or computer part -- but it's more that I tend to hold onto stuff just in case I need it in the future, and usually when i could use it to fix an issue I'm having, it's already obsolete. I just have to remind myself that technology changes so quickly that it really isn't worth it to hold onto, say memory cards.