What are differences between bradycardia and tachycardia?

Heart Rate: Tachycardia is a heart rate greater than 100 beats per minute. Bradycardia is a heart rate less than 60 beats per minute.

Palpitations: Palpitations are extra heart beats felt as an unpleasant sensation. Tachycardia is rapid heart rate and bradycardia is a slow heart rate.

Fast versus slow: Bradycardia & tachycardia just mean slow & fast heart rate, respectively. Each does not necessarily indicate a problem and can be normal & appropriate. For example, sinus tachycardia is a normal response to exercise or fear. If your hr does not increase with exercise, that can be a major problem. Also, bradycardia during rest (e.g., during sleep) can be normal too, especially in an athlete.