What does hoarding look like? You can read a definition of hoarding, but that's words on a page. To get a very real sense of the issue, you can watch the A&E show Hoarders - or you can read this book.
Stuff is filled with vivid vignettes of hoarding situations, told with insight and compassion. You can read a powerful excerpt in Discover Magazine. Here's another, about a man named Ralph:
He showed me a piece of an old Venetian blind, vintage 1950. The rest of the blind had been discarded, though not by Ralph. "Most people would throw this out," he proudly told me. "Not me." He described how it connected to the rest of the blind and how it could be repaired. He insisted that there was someone who needed just such a piece. For most of us, this would not be a sufficient reason for keeping it. For Ralph and many other people with hoarding problems, it is more than sufficient. ...There's also a lot of information to help in understanding hoarding - and because it's tied into the stories of specific people, the book is a fascinating read, rather than a dry textbook.
His home contained thousands of newspapers stacked neatly in piles, some as tall as he was and threatening to collapse as he added to them. To clear space, he moved some to his garage, where they grew wet and moldy. Still he couldn't part with them.
Good Book: Digging Out (recommended by the authors of Stuff)