"Weeding" is a term used by librarians to refer to selecting books to be removed from the collection. This can be a contentious topic, as the thought of "getting rid of books" seems sacrilegious in a library setting. But we're not getting rid of books; we're just moving them elsewhere. And we're doing it for the health of the collection.
Much like how an overgrown forest is not a healthy forest (for those of you not from the southwest, all the tiny, short trees become excellent fuel for forest fires), a cramped library is not a healthy library. We get new books every month, and with a finite amount of space on our shelves, that means we have to weed other books to make room. Not only do library shelves need enough space to hold incoming books, they need enough room to account for books which are currently checked out and will be reshelved when they return.
In addition to this is the fact that cramped shelves cause information overload if you're browsing for some non-specified book. There's just too much to look at, so finding what you're looking for can be a headache. And this is to say nothing of the fact that the spot on the end of each shelf where the weeded books once were can be used to display face-out books, which are statistically more likely to be checked out, because we all, in fact, judge books based on their covers. All in all, empty shelf space is a good thing.
I Fight for the User!
But who gets to decide what books are removed? You do! The basic policy of weeding is to remove books which are no longer circulating--that is, no longer being read by you, the user (which is a fancy librarian word for a library patron, which is an even fancier way of saying customer).
Note that this isn't the same as weeding old books. A brand new book that was checked out once, and then sat, unread, for five years, will be weeded, while a century old book that still gets checked out again and again will stay. The new book wasn't what that particular library's users wanted or needed, but the old book proved its mettle. Weeding comes down to trusting the patrons and keeping what they want in the library, and recognizing that a book that isn't being read isn't doing anyone any good.
Reduce, Reuse, Recycle
But what happens to those poor little weeded books? The vast majority of them are given back to the community, either in the library's book sale, which helps generate funds to buy new materials or do events, or as free books. While a book may not be popular enough to circulate well in a library, it's not unusual for a sale or free book to find that one unique person who loves it and gives it a "forever home". I once weeded an entire set of The Saint by Leslie Charteris, not one of which had been checked out in years, only for a woman to come asking if there were any more, because she had taken the whole series home and loved it so much!
Of course, there are exceptions. Some books are out-of-date, such as test manuals for standardized tests that no longer exist, or road atlases of ever-expanding cities. Some outdated books can actually be a little hazardous, such as ancient manuals on wiring and electricity. And some used up all their luck being published in the first place, like The Complete Beauty Bible, which is an inch and a half thick, all text about beauty treatments and supplies, but no pictures (this is a real book that I have seen with my own eyes. The text was small, and the entries, comprehensive. It really was The Complete Beauty Bible!). Books like these--I'll admit--are disposed of, either by being recycled into lovely book art, or thrown away outright. These books have served their purpose, and can now rest in Book Valhalla, or Book Heaven, or the Summerlands, or wherever it is books go when they die.
Rest in peace, little books. Rest in peace.