Hematuria is a medical term describing the presence of blood in the urine.
Gross hematuria means the urine is visibly bloody; this indicates severe internal bleeding (usually a hemorrhage in the kidneys or bladder caused by problems such as trauma, infectious disease, or mechanical damage from a large kidney stone).
Microscopic hematuria means the blood cannot be seen with the naked eye, but can be detected with a microscope or with chemical tests. This condition also indicates problems such as kidney damage.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.