Collection development is library jargon for deciding what materials (books, videos, CD-ROMs, and other formats) to buy for a library's collection. It also often includes deciding what materials to weed (get rid of) or to shift (from the reference collection to the circulating collection, for example).
My approach to collection development
First and foremost, my goal is to help build the library's collection so that it reflects the needs of its library users (in academic libraries, this means supporting classes taught at that institution). In academia, I prefer to work with faculty in other departments whenever possible. When it isn't possible to do that, I keep in mind the kinds of questions people ask at the reference desk (especially questions that were hard to answer with the resources in our library) and I consult college course guides to keep track of what classes are being offered.
At Pierce Community College
Library faculty make group decisions about most purchases at Pierce. Collection development is a very democratic process.
At Highline Community College
I was responsible for selecting books in the following subject areas: Chiropractic studies, Computer Science, and Languages/Literature.