Below is taken from: http://www.4hcm.org/hcm/overview/3005-what-is-hypertrophic-cardiomyopathy-hcm.html
The main feature of hypertrophic cardiomyopathy is an excessive thickening of the heart muscle (hypertrophy literally means to thicken). Thickening is seen in the ventricular septal measurement (normal range .08-1.2cm), and in weight. In HCM, septal or wall measurements may be in the range of 1.3cm to 6.0+cm anywhere in the left ventricle. Heart muscle may also thicken in normal individuals as a result of high blood pressure or prolonged athletic training. Furthermore, there is a fine line between an athletic heart, hypertensive heart disease and a heart with HCM.
In Hypertrophic Cardiomyopathy (HCM), the muscle thickening occurs without an obvious cause. In addition, microscopic examination of the heart muscle in HCM is abnormal. The normal alignment of muscle cells is absent and this abnormality is called “myocardial disarray”.
This is also a good link to get a medical perspective of the condition: http://www.youtube.com/watch?v=gOFk_uJURpE