Glycosuria (gly-koh-SOOR-ee-ah) is the presence of glucose in the urine.
Glucose, the body's fuel, is produced by digestion and launched into the blood. At this point, the body tissues can only reach this glucose to feed on it if there is insulin present. In diabetics, insulin may be inadequate or not properly used, and the glucose continues to build up in the blood (see hyperglycemia) until the renal threshold for glucose is reached. At this point, the kidneys start to filter excess glucose from the blood and excrete it in the urine, a condition known as glycosuria.
Glucose is filtered through the glomerulus in the kidney; for non-diabetics, all of the glucose is re-absorbed by the renal (kidney) tubules. This means there would be nothing present for a glucose urine test to detect. With diabetes, the kidney tubules are unable to handle and process the amount of glucose they're being presented with, so the glucose winds up in the urine and can be detected in urine glucose testing. 
Glycosuria makes the urine "sweet" (the origin of the term diabetes mellitus), and can promote urinary tract infections  and nephropathy. It also leads to the kidneys demanding more water from the tissues, leading to the classic diabetic symptoms of polyuria and polydipsia, as well as possible dehydration. A heavy presence of glucose in the urine alters the specific gravity of it--increasing its value by 0.010 units due to the amount of glucose present in it.