Stroke volume is the amount of blood expelled from the left ventricle in one heartbeat (ventricular contraction). This is usually around 70ml of blood.
The Frank-Starling law of the heart states that the more the heart is filled, the stronger the force of contraction. Cardiac filling is a function of venous return and left ventricular end diastolic volume (LVEDV), while a stronger force of contraction leads to a bigger stroke volume. Thus, stroke volume is proportional to left ventricular end diastolic volume (LVEDV) and this relationships can be plotted as a Starling curve.
Stroke volume is also influenced by afterload, the pressure against which the left ventricle expels blood, in practice the mean arterial pressure (MAP). While increasing afterload does decrease stroke volume, this effect is fairly minimal for healthy hearts and only becomes significant with extreme hypertension (malignant hypertension) or where cardiac function is compromised.
Increasing heart rate does decreases stroke volume as there is less time to expel blood during the shortened systole. This effect is significant, however, at higher heart rates but is balanced by a reflex increase in inotropy, called the Treppe effect, which ensures that stroke volume remains constant despite physiological variations in heart rate.