The haematocrit ('hematocrit' in American English) is the percentage of the blood that is made up of red blood cells. The rest of the blood consists of plasma and the other cells - leukocytes and platelets.
The haematocrit is generally a calculated value, determined after other variables have been established. (For instance, a commonly-used formula multiplies the average red blood cell volume by the red blood cell count and multiplies this all by 0.1.)
Normal ranges are roughly 40-50% for men, and 35-45% for women. Why the lower reading for women? It is probably due to the fact that women menstruate once a month for much of their lives. Bleeding is one cause of a lowered haematocrit, for two reasons. Firstly, one of the body's initial responses to the decreased blood volume is to increase water reaborption at the kidneys, thus diluting the blood slightly. Secondly, continued low grade blood loss can lead to a mild anaemia, which is also a cause of a low haematocrit, for much the same reason.
Haematocrit can be used to help determine the type of anaemia in a patient, the severity an illness and so on.
Another related measurement is packed cell volume (PCV), which is an older, slightly less accurate measurement of the same thing. It is calculated by centrifuging the blood first, which separates the blood into its components - red blood cells, a small 'buffy coat' of leukocytes and platelets, and plasma. Divide the red cell portion's height by the total height, and you have the PCV, a fair estimate of the haematocrit.