Use the word bedding when you're talking about sheets, pillowcases, blankets, or any other coverings for a bed. If you switch from a twin to a queen-sized bed, you'll need new bedding.
When a college student moves into a dorm room she brings her own bedding, and when your grandmother gets her guest room ready for a visitor, she might wash all the bedding. Bedding also means the straw, hay, or other materials that a farm animal sleeps on — or the shredded paper you use to line your gerbil's cage. It comes from the Old English word beddinge, "bed covering," from bedd, "bed" or "resting place."