Afterload

Ventricular systole. (Red arrow is path from left ventricle to aorta. Afterload is largely dependent upon aortic pressure.
In cardiac physiology, afterload is the tension produced by a chamber of the heart in order to contract. If the chamber is not mentioned, it is usually assumed to be the left ventricle.
Afterload can also be described as the pressure that the chamber of the heart has to generate in order to eject blood out of the chamber. Everything else held equal, as afterload increases, cardiac output decreases.
In the case of the left ventricle, the afterload is a consequence of the blood pressure, since the pressure in the ventricle must be greater than the blood pressure in order to open the aortic valve.
Pathology
Disease processes that increase the left ventricular afterload include increased blood pressure and aortic valve disease.
Hypertension (Increased blood pressure) increases the left ventricular afterload because the left ventricle has to work harder to eject blood into the aorta. This is because the aortic valve won't open until the pressure generated in the left ventricle is higher than the elevated blood pressure.
Aortic stenosis increases the afterload because the left ventricle has to overcome the pressure gradient caused by the stenotic aortic valve in addition to the blood pressure in order to eject blood into the aorta. For instance, if the blood pressure is 120/80, and the aortic valve stenosis creates a trans-valvular gradient of 30 mmHg, the left ventricle has to generate a pressure of 150 mmHg in order to open the aortic valve and eject blood into the aorta.
Aortic insufficiency increases afterload because a percentage of the blood that is ejected forward regurgitates back through the diseased aortic valve. This leads to elevated systolic blood pressure.
Mitral regurgitation decreases the afterload. During ventricular systole, the blood can regurgitate through the diseased mitral valve as well as be ejected through the aortic valve. This means that the left ventricle has to work less to eject blood, causing a decreased afterload.
See also
Preload Cardiac output

Afterload