Is there a difference between bradycardia and sinus bradycardia?

Mechanisms vary. Bradycardia simply means slow heart beat. Sinus bradycardia refers to a specific cause for the slow heart beat which is sluggishness of the natural pacemaker cells of the heart (referred to as sinus node). This could be normal in physically fit people or abnormal especially if associated with symptoms of fatigue, shortness of breath or passing out spells.