There are two major groups of heart sounds, they are classified according to their mechanism, i.e. valvular or ventricular filling. Also the slight asynchrony between the two sides of the heart doubles the number of sounds making a total of 12.
Note that, as with heart murmurs, what is heard in the heart is not an isolated event - there may be clues elsewhere in the body which permit the astute clinician to predict sounds even before the stethoscope is placed on the chest.
Low and medium frequency sounds (eg third and fourth heart sounds) are more easily heard with the bell applied lightly to the skin. High frequency sounds (eg first and second heart sounds, opening snap) are more easily heard with a diaphragm.