Heart sounds are the noises generated by the beating heart and the resultant flow of blood through it. Specifically, the sounds reflect the turbulence created when the heart valves snap shut.
There are four major auscultatory sites used for listening to heart sounds. These are the aortic area, pulmonic area, tricuspid area and the mitral area (Apex) (see Fig. 1). 
Fig. 1. Illustration showing the location of the heart valves and the major auscultatory sites.
Cardiac auscultation is performed systematically over the locations described above on the anterior chest wall using the stethoscope’s diaphragm. Switching to the bell side of the diaphragm helps to better hear lower pitched sounds. 
At each auscultation point, concentrate first on identifying the primary heart sounds – S1 and S2. S1 coincides with closure of the mitral and tricuspid valves and the beginning of ventricular systole. It’s most intense over the apex (mitral area for auscultation). S1 is a lower-pitched, more pronounced sound than S2.
S2 indicates closure of the aortic and pulmonic valves and the onset of diastole. It’s best heard in the second intercostal space at the right sternal border (the aortic area for auscultation). S2 is higher-pitched than S1 and has a clipped, closing sound.
For more on innocent (or physiological) heart sounds/murmurs, pathological heart sounds/murmurs and a detailed description of how to listen for heart and breath sounds, I highly recommend Moriarty Mary-Beth’s (RN, BSN) article; Heart and breath sounds: Listening with skill.