The glomerulus is the beginning section of the nephron, in the kidney, made of a capilliary which is wound in a tight knot structure. The glomerulus is surrounded by a cup-like structure known as the Bowmans' Capsule. The glomerulus is the first stage of filtration for the blood as it is entering the kidney. Blood from the afferent arteriole enters the glomerulus and is put under high amounts of pressure to force molecules out of the blood stream and into the Bowmans' Capsule as the first stage of ultrafiltration. Together, the glomerulus and the Bowmans' Capsule form what is known as the renal corpuscle.
Grant's Atlas of Anatomy, Thirteenth Edition, 2001. A.M.R.Agur, A.F.Dalley.