What is blood pressure?

Blood pressure is the pressure in the blood due to the beating heart.

There are two types of blood pressure: systolic and diastolic.

The systolic blood pressure corresponds to the pressure of the blood when the heart has imparted the maximum pressure.

The diastolic blood pressure is the pressure when the heart is in the resting phase.A more detailed description can be found here.

Information Source: http://www.medphys.ucl.ac.uk/teaching/undergrad/projects/2003/group_03/why.html