A calcium channel blocker is any pharmaceutical (or for that matter, any chemical), which blocks the passage of calcium through its biological channels. Their primary use is to reduce blood pressure, but they are also used to treat vasospasm, angina of the chest, to alter the heart rate, and to treat epilepsy. They're particularly effective in elderly patients. However, they pose a higher mortality risk with long term use, and have numerous side effects.
Calcium channels are found in blood vessels and heart muscle. As such, reducing the amount of calcium passing through these channels reduces the strength of muscle contractions. In the heart, this reduces cardiac output and, in blood vessels, allows them to dilate. Blood pressure drops as a result of a combination of these two factors. This also allows the heart to work more efficiently to pump blood, placing less of a load on its muscles, preventing low oxygen levels.
Calcium channel blockers have some advantages over other blood pressure medication. Unlike diuretics, they do not damage the kidneys. Unlike beta blockers, they do not interfere with the operation of the nervous system.
However, calcium channel blockers do have side effects, including dizziness, headaches, flushing, fluid buildup in the legs, change in heart rate (both tachycardia and bradycardia), and constipation.
Calcium channel blocker at Wikipedia