What's the difference between those terms; tachycardia & palpitation?
well, some students misuse the term 'palpitation' and refer it to a condition of 'tachycardia'. In fact, Tachycardia is the situation where the heart beats are increased, while Palpitation is the situation where the heart beats (which may be normal, increased "Tachycardia", or decreased "Bradycardia") is felt by the patient through his chest.

In other words, Tachycardia (or Bradycardia) refers to the number of heart beats, whereas Palpitation refers to feeling of those beats (which in any case is abnormal, except in marked sympathetic activity i.e. Fight, Flight, and Fright).