Palpitations can be caused by cardiac disorders or noncardiac disorders.  Cardiac etiologies are classified by site of origin and include supraventricular (SVT) and ventricular tachycardias (VT), as well as premature atrial and ventricular depolarizations. Cardiac arrhythmias are also common in the setting of structural heart disease.  Noncardiac causes include panic attacks, anxiety disorder, anemia, fever, hyperthyroidism, hypoglycemia, and electrolyte imbalance. Panic disorder may cause palpitations in 20% of patients.  In addition, many patients with palpitations have a history of panic attacks.  Certain psychiatric disorders such as panic attack or anxiety disorder are common causes of palpitations, but these diagnoses should not be accepted as the sole etiologies of palpitations until true arrhythmic causes have been excluded.  
Sinus tachycardia: the most common etiology of palpitations, defined as a heart rate >100 bpm with an identifiable cause such as exercise, stress, or some pathologic state.
Inappropriate sinus tachycardia: defined as a resting heart rate >100 bpm, or an exaggerated heart response to exercise, without an identifiable etiology for the tachycardia such as pregnancy, anemia, or hyperthyroidism. Etiology is unknown but may be due to autonomic nervous system dysfunction or sinus node abnormality. A related syndrome is the postural orthostatic syndrome where inappropriate tachycardia is demonstrated in response to posture on a tilt-table test.
Atrial tachycardia: frequently seen in patients with structural heart disease or in the setting of an acute illness, such as infection. Alternately, it can be described as incessant with symptoms that are less specific such as fatigue and shortness of breath. Incessant atrial tachycardia can sometimes lead to a tachycardia-induced cardiomyopathy.
Atrial flutter: commonly seen in patients with atrial fibrillation.
Atrial fibrillation: frequently seen in the setting of CHF, valvular heart disease, hypertrophic cardiomyopathy, thyroid disease, and pulmonary disease. Very common in older people, with nearly 10% of those aged >75 years having atrial fibrillation.
Atrioventricular nodal reentrant tachycardia: can occur at any age, although most commonly presents in the late 20s or early 30s.  Palpitations typically have an abrupt initiation and termination.
Atrioventricular reentrant tachycardia: a reentrant SVT using an anomalous bypass tract between the atria and ventricles. When evidence of this bypass tract is seen as a delta wave on the ECG, it is referred to as Wolff-Parkinson-White syndrome; however, it is important to realize that this condition can be present without seeing delta waves on the baseline ECG. It typically presents in the teenage years, is more frequently seen in males, and is associated with a slight increase in incidence of sudden cardiac death.
Reentrant VT due to cardiomyopathy (most often due to CAD and prior infarction).
Brugada syndrome: a familial syndrome that has an associated risk of sudden cardiac death.
Long QT syndrome (LQTS): palpitations are due to episodes of nonsustained polymorphic VT, often with associated syncope. There are both congenital and acquired LQTSs, often associated with sudden cardiac death. In the familial syndromes, classic triggers exist for episodes, which depend on the underlying genotype. In LQTS 1, episodes occur during exercise. In LQTS 2, they often occur with arousal from sleep or during emotional stress. The QT prolongation may also be acquired, and it is essential to evaluate the patient's medications for drugs that may prolong the QT interval, such as macrolide and fluoroquinolone antibiotics, or certain antipsychotics.
Idiopathic VT: several different types of idiopathic VTs, classified by site of origin. These include right ventricular outflow tract VT, left ventricular outflow tract VT, and left ventricular idiopathic VT, sometimes referred to as Belhassen VT or verapamil-sensitive VT, which originates from the left ventricular myocardium.
Other cardiac causes
Ventricular premature contractions (VPCs): one of the most common etiologies of palpitations; can be seen in normal hearts as well as in patients with structural heart disease.
Atrial premature contractions (APCs): one of the most common etiologies of palpitations; can be seen in normal hearts as well as in patients with structural heart disease.
Myocardial infarction: VT is often seen in the acute setting. A myocardial scar from a previous infarction predisposes a patient to VT due to reentry, causing palpitations. Patients with prior MI, especially those that develop CHF, are also especially prone to developing atrial fibrillation and flutter.   
Hypertrophic obstructive cardiomyopathy: a familial syndrome marked by LVH that can present with a variety of arrhythmias that lead to palpitations, as well as dyspnea on exertion, near-syncope, and syncope. The arrhythmias can be supraventricular such as atrial fibrillation, VPCs, or ventricular (typically nonsustained VT).  
Congenital heart defects: repaired or unrepaired atrial septal defects, ventricular septal defects, and tetralogy of Fallot can lead to VT by reentry around surgical scars or atrial tachyarrhythmias due to significant atrial enlargement.   
Cardiac surgery: any cardiac surgery that involves an incision, such as cannulation of the atria for initiation of cardiopulmonary bypass, or exposure of the mitral valve, results in scarring that may become a substrate for a reentrant tachycardia.
Panic disorder 
Anxiety disorder 
Hyperthyroidism: can cause sinus tachycardia or atrial fibrillation 
Pheochromocytoma: palpitations associated with sweating and headache 
Alcohol use: binge drinking in particular may lead to atrial fibrillation 
Drugs: drugs that prolong the QT interval may precipitate polymorphic VT 
Caffeine use: may increase frequency of VPCs and APCs 
Electrolyte imbalance: electrolyte abnormalities (particularly hypokalemia and hypomagnesemia) predispose patients to many arrhythmias from APCs and VPCs to atrial fibrillation and often incite and/or contribute to VT.