Blood pressure is the force, or "tension," of blood pressing against artery walls. If your arteries constrict (get narrower) or if your heart pumps out more blood, the force increases. A simple way to think about blood pressure is to compare it to water in a garden hose. If you step on the hose, the pressure builds up behind your foot. Cranking up the faucet to full flow also increases the pressure.
Blood pressure that is too high (called hypertension) can damage not only the blood vessels themselves (e.g., bulging vessels, called aneurysms) but also organs such as the heart (e.g., heart attack, heart failure), brain (e.g., stroke, memory problems), kidneys (e.g., renal failure or insufficiency), and eyes (e.g., vision loss). Hypertension usually develops gradually over the years. Most often there is no specific cause of hypertension (this is called essential or primary hypertension). Rarely, sudden high blood pressure is triggered by a medication, kidney disease, a tumor or some other condition (this is called secondary hypertension).
Factors that increase your chances of hypertension include: age; family history; being African-American; being overweight; smoking; heavy alcohol use; lack of physical activity; high-salt diet; conditions such as diabetes or high cholesterol.
Note: A serious type of hypertension that occurs in the pulmonary arteries, which carry blood to the lungs, is called pulmonary hypertension and is treated differently.