The Hogwarts broomshed is where the school brooms for Hogwarts are stored.
It also seems to be where at least some of the brooms that personally belong to individual students are kept. We know that Harry keeps his Nimbus 2000 there (PS13), although later his Firebolt broom stays in his Gryffindor dormitory room (PA13, PA15, GF20).
Lexicon list of Broom Manufacturers
From the Web
Harry Potter Wiki: Broomshed
SnitchSeeker RP game: The Broomshed
WizardingWorld.com (Pottermore) features: