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Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. When used without further specification, "blood pressure" usually refers to the arterial pressure of the systemic circulation. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure.[1] The mean BP, due to pumping by the heart and resistance to flow in blood vessels, decreases as the circulating blood moves away from the heart through arteries. Blood pressure drops most rapidly along the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins.[2] Gravity, valves in veins, and pumping from contraction of skeletal muscles are some other influences on BP at various places in the body.
The measurement blood pressure without further specification usually refers to the systemic arterial pressure measured at a person's upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper arm's major blood vessel that carries blood away from the heart. A person's BP is usually expressed in terms of the systolic pressure over diastolic pressure (mmHg), for example 140/90.
SOURCE:  wikipedia 

Systolic- the force/resistances against vs. Diastolic- the relaxation of.

Systolic is the greatest amount of pressure exerted by the heart again the walls of the vessels, diastolic is the least amount of pressure exerted by the heart against the walls of the vessels.... I had to memorize this for nursing school..........